LT1074/LT1076 Step-Down Switching Regulator tions allow this device to be used as a positive-to-negative FEATURES converter, a negative boost converter, and as a flyback 5A On-Board Switch (LT1074) converter. The switch output is specified to swing 40V 100kHz Switching Frequency below ground, allowing the LT1074 to drive a tapped- Greatly Improved Dynamic Behavior inductor in the buck mode with output currents up to 10A. Available in Low Cost 5 and 7-Lead Packages Only 8.5mA Quiescent Current The LT1074 uses a true analog multiplier in the feedback Programmable Current Limit loop. This makes the device respond nearly instanta- Operates Up to 60V Input neously to input voltage fluctuations and makes loop gain Micropower Shutdown Mode independent of input voltage. As a result, dynamic behav- ior of the regulator is significantly improved over previous APPLICATI S designs. Buck Converter with Output Voltage Range of 2.5V On-chip pulse by pulse current limiting makes the LT1074 to 50V nearly bust-proof for output overloads or shorts. The input Tapped-Inductor Buck Converter with 10A Output voltage range as a buck converter is 8V to 60V, but a self- at 5V boot feature allows input voltages as low as 5V in the Positive-to-Negative Converter inverting and boost configurations. Negative Boost Converter Multiple Output Buck Converter The LT1074 is available in low cost TO-220 or TO-3 packages with frequency pre-set at 100kHz and current DESCRIPTIO limit at 6.5A (LT1076 = 2.6A). A 7-pin TO-220 package is The LT1074 is a 5A (LT1076 is rated at 2A) monolithic also available which allows current limit to be adjusted bipolar switching regulator which requires only a few down to zero. In addition, full micropower shutdown can external parts for normal operation. The power switch, all be programmed. See Application Note 44 for design oscillator and control circuitry, and all current limit com- details. ponents, are included on the chip. The topology is a classic A fixed 5V output, 2A version is also available. See LT1076-5. positive buck configuration but several design innova- TYPICAL APPLICATI Buck Converter Efficiency Basic Positive Buck Converter LT1074 L1** 100 50 H (LT1074) µ 100 H (LT1076) µ VOUT = 12V, VIN = 20V 5V VIN VSW 90 5A *USE MBR340 FOR LT1076 10V TO 40V **COILTRONICS #50-2-52 (LT1074) R1 MBR745* LT1074 #100-1-52 (LT1076) 2.8k 80 PULSE ENGINEERING, INC. 1% VOUT = 5V, VIN = 15V FB #PE-92114 (LT1074) GND VC #PE-92102 (LT1076) 70 HURRICANE #HL-AK147QQ (LT1074) R2 R3 #HL-AG210LL (LT1076) 2.21k 2.7k
L = 50 µ H TYPE 52 CORE RIPPLE CURRENT RATING e" IOUT / 2 60 1% + + C3 C2 C1 DIODE = MBR735 200 F 0.01 F µ µµ 500 F 50 25V 0 1 2 3 4 5 6 OUTPUT LOAD CURRENT (A) LT1074 " TA01 LT1074 " TPC27 1 EFFICIENCY (%) U O U U O LT1074/LT1076 ABSOLUTE AXI U RATI GS Input Voltage ILIM Pin Voltage (Forced) ............................................ 5.5V LT1074/ LT1076 .................................................. 45V Maximum Operating Ambient Temperature Range LT1074HV/76HV.................................................. 64V LT1074C/76C, LT1074HVC/76HVC ............ 0°C to 70°C Switch Voltage with Respect to Input Voltage LT1074I/76I, LT1074HVI/76HVI ............. 40°C to 85°C LT1074/ 76 .......................................................... 64V LT1074M/76M, LT1074HVM/76HVM ... 55°C to 125°C LT1074HV/76HV.................................................. 75V Maximum Operating Junction Temperature Range Switch Voltage with Respect to Ground Pin (VSW Negative) LT1074C/76C, LT1074HVC/76HVC .......... 0°C to 125°C LT1074/76 (Note 6) ............................................. 35V LT1074I/76I, LT1074HVI/76HVI ........... 40°C to 125°C LT1074HV/76HV (Note 6).................................... 45V LT1074M/76M, LT1074HVM/76HVM ... 55°C to 150°C Feedback Pin Voltage..................................... 2V, +10V Maximum Storage Temperature................ 65°C to 150°C Shutdown Pin Voltage (Not to Exceed VIN) .............. 40V Lead Temperature (Soldering, 10 sec) ..................... 300°C PACKAGE/ORDER I FOR ATIO FRONT VIEW ORDER PART ORDER PART 5 VIN NUMBER NUMBER 4 VSW BOTTOM VIEW 3 GND 2 VC LT1076CQ VC LT1074CK VIN 1 FB/SENSE LT1074HVCK 1 Q PACKAGE 2 CASE IS GND 5-LEAD PLASTIC DD LT1074MK 3 4 LT1076: ¸JC = 4°C/W, ¸JA = 30°C/W* LT1074HVMK FB VSW FRONT VIEW LT1076CK K PACKAGE, 4-LEAD TO-3 METAL CAN 7 SHDN LT1076CR LT1076HVCK 6 VC LT1074: ¸JC = 2.5°C/W, ¸JA = 35°C/W 5 FB/SENSE LT1076HVCR LT1076: ¸JC = 4°C/W, ¸JA = 35°C/W LT1076MK 4 GND 3 ILIM LT1076HVMK 2 VSW 1 VIN R PACKAGE 7-LEAD PLASTIC DD FRONT VIEW LT1076: ¸JC = 4°C/W, ¸JA = 30°C/W* LT1074CT 5 VIN 4 VSW LT1074HVCT FRONT VIEW 3 LT1074CY GND 7 SHUTDOWN 2 VC LT1074IT 6 VC 1 FB LT1074HVCY 5 FB 4 GND LT1074HVIT 3 ILIM LT1074IY T PACKAGE, 5-LEAD T0-220 2 VSW 1 VIN LT1076CT LEADS ARE FORMED STANDARD FOR LT1074HVIY STRAIGHT LEADS, ORDER FLOW 06 Y PACKAGE, 7-LEAD TO-220 LT1076HVCT LT1076CY LT1074: ¸JC = 2.5°C/W, ¸JA = 50°C/W LT1074: ¸JC = 2.5°C/W, ¸JA = 50°C/W LT1076: ¸JC = 4°C/W, ¸JA = 50°C/W LT1076IT LT1076: ¸JC = 4°C/W, ¸JA = 50°C/W LT1076HVCY * Assumes package is soldered to 0.5 IN2 of 1 oz. copper over internal ground plane or over back side plane. ELECTRICAL CHARACTERISTICS Tj = 25°C, VIN = 25V, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS Switch On Voltage (Note 1) LT1074 ISW = 1A, Tj e" 0°C 1.85 V ISW = 1A, Tj < 0°C 2.1 V ISW = 5A, Tj e" 0°C 2.3 V ISW = 5A, Tj < 0°C 2.5 V LT1076 ISW = 0.5A 1.2 V ISW = 2A 1.7 V 2 W W W U W U U LT1074/LT1076 ELECTRICAL CHARACTERISTICS Tj = 25°C, VIN = 25V, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS Switch Off Leakage LT1074 VIN d" 25V, VSW = 0 5 300 µA VIN = VMAX, VSW = 0 (Note 7) 10 500 µA LT1076 VIN = 25V, VSW = 0 150 µA VIN = VMAX, VSW = 0 (Note 7) 250 µA Supply Current (Note 2) VFB = 2.5V, VIN d" 40V 8.5 11 mA 40V < VIN < 60V 912 mA VSHUT = 0.1V (Device Shutdown) (Note 8) 140 300 µA Minimum Supply Voltage Normal Mode 7.3 8 V Startup Mode (Note 3) 3.5 4.8 V Switch Current Limit (Note 4) LT1074 ILIM Open 5.5 6.5 8.5 A RLIM = 10k (Note 5) 4.5 A RLIM = 7k (Note 5) 3 A LT1076 ILIM Open 2 2.6 3.2 A RLIM = 10k (Note 5) 1.8 A RLIM = 7k (Note 5) 1.2 A Maximum Duty Cycle 85 90 % Switching Frequency 90 100 110 kHz Tj d" 125°C 85 120 kHz Tj > 125°C 85 125 kHz VFB = 0V through 2k&! (Note 4) 20 kHz Switching Frequency Line Regulation 8V d" VIN d" VMAX (Note 7) 0.03 0.1 %/V Error Amplifier Voltage Gain (Note 6) 1V d" VC d" 4V 2000 V/V Error Amplifier Transconductance 3700 5000 8000 µmho Error Amplifier Source and Sink Current Source (VFB = 2V) 100 140 225 µA Sink (VFB = 2.5V) 0.7 1 1.6 mA Feedback Pin Bias Current VFB = VREF 0.5 2 µA Reference Voltage VC = 2V 2.155 2.21 2.265 V Reference Voltage Tolerance VREF (Nominal) = 2.21V Ä… 0.5 Ä… 1.5 % All Conditions of Input Voltage, Output Ä… 1 Ä… 2.5 % Voltage, Temperature and Load Current Reference Voltage Line Regulation 8V d" VIN d" VMAX (Note 7) 0.005 0.02 %/V VC Voltage at 0% Duty Cycle 1.5 V Over Temperature 4 mV/°C Multiplier Reference Voltage 24 V Shutdown Pin Current VSH = 5V 510 20 µA VSH d" VTHRESHOLD (E"2.5V) 50 µA Shutdown Thresholds Switch Duty Cycle = 0 2.2 2.45 2.7 V Fully Shut Down 0.1 0.3 0.5 V Thermal Resistance Junction to Case LT1074 2.5 °C/W LT1076 4.0 °C/W The denotes the specifications which apply over the full operating temperature Note 4: Switch frequency is internally scaled down when the feedback pin voltage range. is less than 1.3V to avoid extremely short switch on times. During testing, VFB is adjusted to give a minimum switch on time of 1µs. Note 1: To calculate maximum switch on voltage at currents between low and high conditions, a linear interpolation may be used. RLIM 1k RLIM 1k Note 5: ILIM H" (LT1074), ILIM H" (LT1076). Note 2: A feedback pin voltage (VFB) of 2.5V forces the VC pin to its low clamp 2k 5.5k level and the switch duty cycle to zero. This approximates the zero load condition Note 6: Switch to input voltage limitation must also be observed. where duty cycle approaches zero. Note 7: VMAX = 40V for the LT1074/76 and 60V for the LT1074HV/76HV. Note 3: Total voltage from VIN pin to ground pin must be e" 8V after startup for Note 8: Does not include switch leakage. proper regulation. 3 LT1074/LT1076 I BLOCK DAGRA INPUT SUPPLY LT1074 320 µ A 10 µ A 0.3V + 6V 500&! 6V TO ALL µ-POWER REGULATOR SHUTDOWN CIRCUITRY AND BIAS
CURRENT LIMIT 0.04 COMP + CURRENT 2.35V LIMIT C2 + 250 &! SHUTDOWN
ILIM* SHUTDOWN* 4.5V 10k FREQ SHIFT R 100kHz G1 R/S S Q OSCILLATOR LATCH SYNC R 3V(p-p) VIN + 400 &! 15&! Z + C1 ANALOG A1 MULTIPLIER ERROR X XY PULSE WIDTH AMP Z COMPARATOR 2.21V SWITCH Y OUTPUT (VSW) FB VC 24V (EQUIVALENT) LT1076 0.1&! *AVAILABLE ON PACKAGES WITH PIN COUNTS GREATER THAN 5. 100&! SWITCH OUTPUT (VSW) LT1074 " BD01 4 W LT1074/LT1076 I BLOCK DAGRA DESCRIPTIO A switch cycle in the LT1074 is initiated by the oscillator voltages by feeding the FB signal into the oscillator and setting the R/S latch. The pulse that sets the latch also creating a linear frequency downshift when the FB signal locks out the switch via gate G1. The effective width of this drops below 1.3V. Current trip level is set by the voltage on pulse is approximately 700ns, which sets the maximum the ILIM pin which is driven by an internal 320µA current switch duty cycle to approximately 93% at 100kHz switch- source. When this pin is left open, it self-clamps at about ing frequency. The switch is turned off by comparator C1, 4.5V and sets current limit at 6.5A for the LT1074 and 2.6A which resets the latch. C1 has a sawtooth waveform as one for the LT1076. In the 7-pin package an external resistor input and the output of an analog multiplier as the other can be connected from the ILIM pin to ground to set a lower input. The multiplier output is the product of an internal current limit. A capacitor in parallel with this resistor will reference voltage, and the output of the error amplifier, A1, soft start the current limit. A slight offset in C2 guarantees divided by the regulator input voltage. In standard buck that when the ILIM pin is pulled to within 200mV of ground, regulators, this means that the output voltage of A1 C2 output will stay high and force switch duty cycle to zero. required to keep a constant regulated output is indepen- The Shutdown pin is used to force switch duty cycle to dent of regulator input voltage. This greatly improves line zero by pulling the ILIM pin low, or to completely shut down transient response, and makes loop gain independent of the regulator. Threshold for the former is approximately input voltage. The error amplifier is a transconductance 2.35V, and for complete shutdown, approximately 0.3V. type with a GM at null of approximately 5000µmho. Slew Total supply current in shutdown is about 150µA. A 10µA current going positive is 140µA, while negative slew pull-up current forces the shutdown pin high when left current is about 1.1mA. This asymmetry helps prevent open. A capacitor can be used to generate delayed start- overshoot on start-up. Overall loop frequency compensa- up. A resistor divider will program undervoltage lockout tion is accomplished with a series RC network from VC to if the divider voltage is set at 2.35V when the input is at the ground. desired trip point. Switch current is continuously monitored by C2, which The switch used in the LT1074 is a Darlington NPN (single resets the R/S latch to turn the switch off if an overcurrent NPN for LT1076) driven by a saturated PNP. Special condition occurs. The time required for detection and patented circuitry is used to drive the PNP on and off very switch turn off is approximately 600ns. So minimum quickly even from the saturation state. This particular switch on time in current limit is 600ns. Under dead switch arrangement has no isolation tubs connected to shorted output conditions, switch duty cycle may have to the switch output, which can therefore swing to 40V below be as low as 2% to maintain control of output current. This ground. would require switch on time of 200ns at 100kHz switch- ing frequency, so frequency is reduced at very low output 5 W U LT1074/LT1076 TYPICAL PERFOR A CE CHARACTERISTICS VC Pin Characteristics VC Pin Characteristics Feedback Pin Characteristics 2.0 500 200 400 1.5 150 300 V ADJUSTED FOR 1.0 100 FB V e" 2.5V FB 200 START OF I = 0 AT V = 2V C C FREQUENCY SHIFTING 0.5 50 100 0 0 0 100 0.5 50 H" SLOPE 400k&! 200 1.0 100 300 VFB d" 2V 1.5 150 400 2.0 500 200 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 VOLTAGE (V) VOLTAGE (V) VOLTAGE (V) LT1074 " TPC02 LT1074 " TPC03 LT1074 " TPC01 Shutdown Pin Characteristics Shutdown Pin Characteristics ILIM Pin Characteristics 40 0 100 ° Tj = 25 C 50 30 5 CURRENT FLOWS OUT 0 OF SHUTDOWN PIN ° 20 10 T = 25 C j 50 VIN = 50V 10 15 100 SHUTDOWN THIS POINT MOVES THRESHOLD 0 WITH VIN 20 150 200 10 25 250 20 30 DETAILS OF THIS 300 AREA SHOWN IN 30 35 350 OTHER GRAPH 40 40 400 0 10 20 30 40 50 60 70 80 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 2 1 0 1 2 3 4 5 6 7 8 5 VOLTAGE (V) VOLTAGE (V) VOLTAGE (V) LT1074 " TPC04 LT1074 " TPC05 LT1074 " TPC06 Supply Current 20 18 16 14 DEVICE NOT SWITCHING 12 V = 1V C 10 8 6 4 2 0 0 10 20 30 40 50 60 INPUT VOLTAGE (V) LT1074 " TPC11 6 µ CURRENT ( A) CURRENT (mA) CURRENT (mA) µ µ µ CURRENT ( A) CURRENT ( A) CURRENT ( A) INPUT CURRENT (mA) U W LT1074/LT1076 TYPICAL PERFOR A CE CHARACTERISTICS Reference Voltage vs Supply Current (Shutdown) Temperature Switch On Voltage 300 2.25 3.0 ° T = 25 C j 2.24 250 2.5 2.23 200 2.22 2.0 LT1074 150 2.21 1.5 2.20 100 2.19 LT1076 1.0 50 2.18 2.17 0 0.5 0 10 20 30 40 50 60 50 25 0 25 50 75 100 125 150 0 1 2 3 4 5 6 INPUT VOLTAGE (V) JUNCTION TEMPERATURE (°C) SWITCH CURRENT (A) LT1074 " TPC13 LT1074 " TPC14 LT1074 " TPC28 Reference Shift with Ripple Switching Frequency vs Voltage Error Amplifier Phase and GM Temperature 20 8k 200 120 10 7k 150 115 0 ¸ 6k 100 110 10 TRI WAVE 5k 50 105 20 SQUARE 30 4k 0 100 WAVE GM 40 3k 50 95 50 2k 90 100 60 1k 85 150 70 0 80 80 200 0 20 40 60 80 100 120 140 160 180 200 1k 10k 100k 1M 10M 50 25 0 25 50 75 100 125 150 PEAK-TO-PEAK RIPPLE AT FB PIN (mV) FREQUENCY (Hz) JUNCTION TEMPERATURE ( C) ° LT1074 " TPC16 LT1074 " TPC17 LT1074 " TPC18 Feedback Pin Frequency Shift Current Limit vs Temperature* 8 160 140 7 I PIN OPEN LIM 6 120 5 100 RLIM = 10k&! 80 4 150°C 60 3 55°C RLIM = 5k&! 40 2 25°C 1 20 *MULTIPLY CURRENTS BY 0.4 FOR LT1076 0 0 50 25 0 25 50 75 100 125 150 0 0.5 1.0 1.5 2.0 2.5 3.0 ° JUNCTION TEMPERATURE ( C) FEEDBACK PIN VOLTAGE (V) LT1074 " TPC22 LT1074 " TPC19 7 PHASE ( ) ° µ VOLTAGE (V) "ON" VOLTAGE (V) INPUT CURRENT ( A) µ FREQUENCY (kHz) TRANSCONDUCTANCE ( mho) CHANGE IN REFERENCE VOLTAGE (mV) OUTPUT CURRENT LIMIT (A) SWITCHING FREQUENCY (kHz) U W LT1074/LT1076 PI DESCRIPTIO S VIN PIN The VIN pin is both the supply voltage for internal control "VGND VOUT ( )( ) "VOUT = circuitry and one end of the high current switch. It is 2.21 important, especially at low input voltages, that this pin be bypassed with a low ESR, and low inductance capacitor to To ensure good load regulation, the ground pin must be prevent transient steps or spikes from causing erratic connected directly to the proper output node, so that no operation. At full switch current of 5A, the switching high currents flow in this path. The output divider resistor transients at the regulator input can get very large as should also be connected to this low current connection shown in Figure 1. Place the input capacitor very close to line as shown in Figure 2. the regulator and connect it with wide traces to avoid extra inductance. Use radial lead capacitors. LT1074 FB GND dI LP ( )( ) dt R2 STEP = ISW ESR ( )( ) RAMP = NEGATIVE OUTPUT NODE ISW TON ( )( ) HIGH CURRENT WHERE LOAD REGULATION RETURN PATH C WILL BE MEASURED LT1074 " PD01 LT1074 " PD02 Figure 2. Proper Ground Pin Connection Figure 1. Input Capacitor Ripple LP = Total inductance in input bypass connections FEEDBACK PIN and capacitor. The feedback pin is the inverting input of an error amplifier Spike height (dI/dt " LP) is approximately 2V per which controls the regulator output by adjusting duty inch of lead length for LT1074 and 0.8V per inch for cycle. The non-inverting input is internally connected to a LT1076. trimmed 2.21V reference. Input bias current is typically Step for ESR = 0.05&! and ISW = 5A is 0.25V. 0.5µA when the error amplifier is balanced (IOUT = 0). The Ramp for C = 200µF, TON = 5µs, and ISW = 5A, error amplifier has asymmetrical GM for large input sig- is 0.12V. nals to reduce startup overshoot. This makes the amplifier more sensitive to large ripple voltages at the feedback pin. Input current on the VIN Pin in shutdown mode is the sum 100mVp-p ripple at the feedback pin will create a 14mV of actual supply current (H"140µA, with a maximum of offset in the amplifier, equivalent to a 0.7% output voltage 300µA), and switch leakage current. Consult factory for shift. To avoid output errors, output ripple (P-P) should be special testing if shutdown mode input current is critical. less than 4% of DC output voltage at the point where the output divider is connected. GROUND PIN See the Error Amplifier section for more details. It might seem unusual to describe a ground pin, but in the case of regulators, the ground pin must be connected Frequency Shifting at the Feedback Pin properly to ensure good load regulation. The internal reference voltage is referenced to the ground pin; so any The error amplifier feedback pin (FB) is used to downshift error in ground pin voltage will be multiplied at the output; the oscillator frequency when the regulator output voltage 8 U U LT1074/LT1076 PI DESCRIPTIO S is low. This is done to guarantee that output short circuit SHUTDOWN PIN current is well controlled even when switch duty cycle The shutdown pin is used for undervoltage lockout, mi- must be extremely low. Theoretical switch on time for a cropower shutdown, soft start, delayed start, or as a buck converter in continuous mode is; general purpose on/off control of the regulator output. It controls switching action by pulling the ILIM pin low, which VOUT + VD forces the switch to a continuous off state. Full tON = VIN " f micropower shutdown is initiated when the shutdown pin drops below 0.3V. VD = Catch diode forward voltage ( H" 0.5V) f = Switching frequency The V/I characteristics of the shutdown pin are shown in Figure 4. For voltages between 2.5V and H"VIN, a current At f = 100kHz, tON must drop to 0.2µs when VIN = 25V of 10µA flows out of the shutdown pin. This current and the output is shorted (VOUT = 0V). In current limit, increases to H"25µA as the shutdown pin moves through the LT1074 can reduce tON to a minimum value of H" the 2.35V threshold. The current increases further to H" 0.6µs, much too long to control current correctly for 30µA at the 0.3V threshold, then drops to H"15µA as the VOUT = 0. To correct this problem, switching frequency shutdown voltage falls below 0.3V. The 10µA current is lowered from 100kHz to 20kHz as the FB pin drops source is included to pull the shutdown pin to its high or from 1.3V to 0.5V. This is accomplished by the circuitry default state when left open. It also provides a convenient shown in Figure 3. TO pullup for delayed start applications with a capacitor on OSCILLATOR the shutdown pin. VOUT +2V Q1 When activated, the typical collector current of Q1 in R1 Figure 5, is H" 2mA. A soft start capacitor on the ILIM pin + 2.21V R3 3k ERROR will delay regulator shutdown in response to C1, by AMPLIFIER EXTERNAL VC H"(5V)(CLIM)/2mA. Soft start after full micropower shut- DIVIDER FB down is ensured by coupling C2 to Q1. R2 2.21k 0 LT1074 " PD03 ° Tj = 25 C 5 CURRENT FLOWS OUT Figure 3. Frequency Shifting OF SHUTDOWN PIN 10 15 Q1 is off when the output is regulating (VFB = 2.21V). As SHUTDOWN THRESHOLD the output is pulled down by an overload, VFB will eventu- 20 ally reach 1.3V, turning on Q1. As the output continues to 25 drop, Q1 current increases proportionately and lowers the 30 frequency of the oscillator. Frequency shifting starts when 35 the output is H" 60% of normal value, and is down to its 40 minimum value of E" 20kHz when the output is E" 20% of 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 normal value. The rate at which frequency is shifted is VOLTAGE (V) LT1074 " TPC05 determined by both the internal 3k resistor R3 and the Figure 4. Shutdown Pin Characteristics external divider resistors. For this reason, R2 should not be increased to more than 4k&!, if the LT1074 will be subjected to the simultaneous conditions of high input voltage and output short circuit. 9 µ CURRENT ( A) U U LT1074/LT1076 PI DESCRIPTIO S VIN Hysteresis in undervoltage lockout may be accomplished by connecting a resistor (R3) from the ILIM pin to the 300 µ A 10 µ A shutdown pin as shown in Figure 7. D1 prevents the shutdown divider from altering current limit. SHUTDOWN
R2 C2 OPTIONAL CURRENT LIMIT RESISTOR 0.3V + LT1074 " PD09 *1N4148 TO TOTAL REGULATOR Figure 7. Adding Hysteresis SHUTDOWN LT1074 " PD07 ëÅ‚ öÅ‚ R1 Figure 5. Shutdown Circuitry Trip Point = VTP = 2.35V ìÅ‚1+ R2÷Å‚ íÅ‚ Å‚Å‚ Undervoltage Lockout If R3 is added, the lower trip point (VIN descending) will be Undervoltage lockout point is set by R1 and R2 in Figure the same. The upper trip point (VUTP) will be; 6. To avoid errors due to the 10µA shutdown pin current, R2 is usually set at 5k, and R1 is found from: ëÅ‚ öÅ‚ ëÅ‚ öÅ‚ R1 R1 R1 VUTP = VSH ìÅ‚1+ + 0.8V R2 R3÷Å‚ ìÅ‚ R3÷Å‚ íÅ‚ Å‚Å‚ íÅ‚ Å‚Å‚ VTP VSH () R1= R2 VSH If R1 and R2 are chosen, R3 is given by VTP = Desired undervoltage lockout voltage. VSH = Threshold for lockout on the shutdown VSH 0.8V R1 ()( ) R3 = pin = 2.45V. ëÅ‚ öÅ‚ R1 VUTP VSH ìÅ‚1+ If quiescent supply current is critical, R2 may be increased R2÷Å‚ íÅ‚ Å‚Å‚ up to 15k&!, but the denominator in the formula for R2 should replace VSH with VSH (10µA)(R2). Example: An undervoltage lockout is required such that the output will not start until VIN = 20V, but will continue to operate until VIN drops to 15V. Let R2 = 2.32k. R1 VIN SHUT LT1074 15V 2.35V () R1= 2.32k = 12.5k ( )2.35V R2 GND 5k 2.35 0.8 12.5 ()( ) LT1074 " PD08 R3 = = 3.9k ëÅ‚ öÅ‚ 12.5 Figure 6. Undervoltage Lockout 20 2.35 ìÅ‚1+ 2.32÷Å‚ íÅ‚ Å‚Å‚ 10 U U LT1074/LT1076 PI DESCRIPTIO S from forcing current back into the ILIM pin. To calculate a ILIM PIN value for RFB, first calculate RLIM, then RFB; The ILIM pin is used to reduce current limit below the preset value of 6.5A. The equivalent circuit for this pin is ISC - 0.44* RL ()( ) shown in Figure 8. RFB = RLink&! ( ) 05* 1k&! ISC . L (R ) TO LIMIT VIN CIRCUIT *Change 0.44 to 0.16, and 0.5 to 0.18 for LT1076. 320 µ A D2 Example: ILIM = 4A, ISC = 1.5A, RLIM = (4)(2k) + 1k = 9k Q1 D1 4.3V R1 15 . ( - 0.44 9k&! ) ( ) 8K D3 RFB = 6V 05 9k - 1k - 1.5 . ( ) I LIM LT1047 " PD12 VOUT Figure 8. ILIM Pin Circuit LT1074 FB When ILIM is left open, the voltage at Q1 base clamps at 5V I LIM through D2. Internal current limit is determined by the current through Q1. If an external resistor is connected RFB D2 RLIM 1N4148 between ILIM and ground, the voltage at Q1 base can be reduced for lower current limit. The resistor will have a LT1074 " PD13 voltage across it equal to (320µA) (R), limited to H" 5V Figure 9. Foldback Current Limit when clamped by D2. Resistance required for a given current limit is ERROR AMPLIFIER RLIM = ILIM (2k&!) + 1k&! (LT1074) The error amplifier in Figure 10 is a single stage design RLIM = ILIM (5.5k&!) + 1k&! (LT1076) with added inverters to allow the output to swing above and below the common mode input voltage. One side of As an example, a 3A current limit would require 3A (2k) + the amplifier is tied to a trimmed internal reference voltage 1k = 7k&! for the LT1074. The accuracy of these formulas of 2.21V. The other input is brought out as the FB (feed- is Ä…25% for 2A d" ILIM d" 5A (LT1074) and 0.7A d" ILIM d" 1.8A back) pin. This amplifier has a GM (voltage in to current (LT1076), so ILIM should be set at least 25% above the out ) transfer function of H"5000µmho. Voltage gain is peak switch current required. determined by multiplying GM times the total equivalent Foldback current limiting can be easily implemented by output loading, consisting of the output resistance of Q4 adding a resistor from the output to the ILIM pin as shown and Q6 in parallel with the series RC external frequency in Figure 9. This allows full desired current limit (with or compensation network. At DC, the external RC is ignored, without RLIM) when the output is regulating, but reduces and with a parallel output impedance for Q4 and Q6 of current limit under short circuit conditions. A typical value 400k&!, voltage gain is H" 2000. At frequencies above a few for RFB is 5k&!, but this may be adjusted up or down to set hertz, voltage gain is determined by the external compen- the amount of foldback. D2 prevents the output voltage sation, RC and CC. 11 U U LT1074/LT1076 PI DESCRIPTIO S 5.8V Q4 90 µ A µ 90 A Q3 50 µ A VC D1 EXTERNAL FREQUENCY Q1 Q2 FB COMPENSATION 50 µ A 90 µ A X1.8 D2 RC Q6 2.21V 140 µ A CC 300&! LT1074 " PD11 ALL CURRENTS SHOWN ARE AT NULL CONDITION Figure 10. Error Amplifier The error amplifier has asymmetrical peak output current. Gm AV = at midfrequencies Q3 and Q4 current mirrors are unity gain, but the Q6 mirror 2Ä„ " f " CC has a gain of 1.8 at output null and a gain of 8 when the FB AV = Gm" RC at high frequencies pin is high (Q1 current = 0). This results in a maximum positive output current of 140µA and a maximum negative Phase shift from the FB pin to the VC pin is 90° at mid- (sink) output current of E" 1.1mA. The asymmetry is frequencies where the external CC is controlling gain, then deliberate it results in much less regulator output over- drops back to 0° (actually 180° since FB is an inverting shoot during rapid start-up or following the release of an input) when the reactance of CC is small compared to RC. output overload. Amplifier offset is kept low by area scaling The low frequency pole where the reactance of CC is Q1 and Q2 at 1.8:1. equal to the output impedance of Q4 and Q6 (rO), is Amplifier swing is limited by the internal 5.8V supply for 1 positive outputs and by D1 and D2 when the output goes fPOLE = r0 H" 400k&! 2Ä„ " r0 " C low. Low clamp voltage is approximately one diode drop (H" 0.7V 2mV/°C). Although fPOLE varies as much as 3:1 due to rO variations, mid-frequency gain is dependent only on GM, which is Note that both the FB pin and the VC pin have other internal specified much tighter on the data sheet. The higher connections. Refer to the frequency shifting and frequency zero is determined solely by RC and CC. sychronizing discussions. 1 fZERO = 2Ä„ " RC " CC 12 U U LT1074/LT1076 TYPICAL APPLICATI S Tapped-Inductor Buck Converter L2 L1* µ 5 H VOUT VIN VIN VSW 5V, 10A 3 1