LT1572 100kHz, 1.25A Switching Regulator with Catch Diode U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ Catch Diode Included in Package Wide Input Voltage Range: 3V to 30V Low Quiescent Current: 6mA Internal 1.25A Switch Very Few External Parts Required Self-Protected Against Overloads Operates in Nearly All Switching Topologies Shutdown Mode Draws Only 50µA Typical Current Can Be Externally Synchronized UO APPLICATI ■ ■ ■ ■ S The LT ®1572 is a 1.25A 100kHz monolithic switching regulator with on-board switch and catch diode included in one package. It combines an LT1172 with a 1A Schottky catch diode. The LT1572 can be operated in all standard switching configurations, including boost, buck, SEPIC, flyback, forward, inverting and “Cuk”. All necessary control, oscillator and protection circuitry is included on the die with the high efficiency switch. This makes the part extremely easy to use and provides “bustproof” operation similar to that obtained with 3-pin linear regulators. The LT1572 operates with supply voltages from 3V to 30V and draws only 6mA quiescent current. It can deliver load power up to 15W with no external power devices. By utilizing a current mode switching technique, the LT1572 achieves excellent response to load and line transients. 3.3V-to-5V and 5V-to-12V Boost Converters Negative-to-Positive Converter SEPIC Converter (Input Can Be Greater or Less Than Output) Battery Charger The LT1572 has many unique features not found on the more difficult to use control chips presently available. It uses adaptive anti-sat switch drive to allow very wide ranging load currents with no loss in efficiency. An externally activated shutdown mode reduces total supply current to 50µA typical for standby operation. External synchronizing of switching frequency is possible, with a range of 120kHz to 160kHz. , LTC and LT are registered trademarks of Linear Technology Corporation. UO TYPICAL APPLICATI Boost Converter Efficiency 5V-to-12V Boost Converter 100 L1* 50µH 9 11 2 15 † VSW ANODE VIN 3 CATHODE† 14 + LT1572 C3 100µF 10V FB E1 10 E2 12 GND 4, 13 + VC 5 R3 1k C1 1µF 12V 0.25A R1 10.7k 1% R2 1.24k 1% C2** 100µF 16V 80 70 60 50 LT1572 • TA01 BOOST CONVERTER VIN = 5V VOUT = 12V 90 EFFICIENCY (%) VIN 4.5V TO 10V *COILTRONICS CTX50-2 **AVX TPS OR SPRAGUE 593D † ALWAYS CONNECT BOTH ANODE (2, 15) AND CATHODE (3, 14) PINS 0 50 150 200 100 L0AD CURRENT (mA) 250 LT1572 • TA01 1 LT1572 U W W W Supply Voltage (Note 4).......................................... 40V Switch Output Voltage (Note 4) .............................. 60V Feedback Pin Voltage (Transient, 1ms) ................ ±15V Operating Junction Temperature Range Operating .............................................. 0°C to 100°C Short Circuit ......................................... 0°C to 125°C Storage Temperature Range ............... – 65°C to 150°C Lead Temperature (Soldering, 10 sec)................. 300°C DIODE Average Forward Current .......................................... 1A Peak Repetitive Forward Current .............................. 2A Peak Non-Repetitive Forward Current....................... 3A Peak Repetitive Reverse Voltage............................. 20V Continuous (Average) Reverse Voltage .................. 15V Operating Junction Temperature ......................... 125°C Note 1: Minimum effective switch “on” time for the LT1572 (in current limit only) is ≈ 0.6µs. This limits the maximum safe input voltage during an output shorted condition. Buck mode and inverting mode input voltage during an output shorted condition is limited to: R × IL + Vf VIN (max, output shorted) = 15V + t×f buck and inverting mode R = Inductor DC resistance IL = 2.5A Vf = Output catch diode forward voltage at IL t = 0.6µs, f = 100kHz switching frequency ELECTRICAL CHARACTERISTICS CONDITIONS VREF Measured at Feedback Pin VC = 0.8V IB Feedback Input Current PACKAGE/ORDER I FOR ATIO ORDER PART NUMBER TOP VIEW NC 1 ANODE* 2 CATHODE* 3 GND 4 16 NC 15 ANODE* LT1572CS 14 CATHODE* 13 GND VC 5 12 E2 FB 6 11 VSW NC 7 10 E1 NC 8 9 VIN S PACKAGE 16-LEAD PLASTIC SO *ALWAYS CONNECT BOTH ANODE AND BOTH CATHODE PINS TJMAX (REGULATOR) = 100°C TJMAX (DIODE) = 125°C SEE THERMAL MANAGEMENT SECTION FOR θJA Consult factory for Industrial and Military grade parts. Maximum input voltage can be increased by increasing R or Vf. External current limiting such as that shown in AN19, Figure 39, will provide protection up to the full supply voltage rating. C1 in Figure 39 should be reduced to 200pF. Transformer designs will tolerate much higher input voltages because leakage inductance limits rate of rise of current in the switch. These designs must be evaluated individually to assure that current limit is well controlled up to maximum input voltage. Boost mode designs are never protected against output shorts because the external catch diode and inductor connect input to output. VIN = 15V, VC = 0.5V, VFB = VREF, output pin open, unless otherwise noted. SYMBOL PARAMETER Reference Voltage U RATI GS W AXI U ● MIN TYP MAX UNITS 1.224 1.214 1.244 1.244 1.264 1.274 V V 350 750 1100 nA nA VFB = VREF ● gm AV Error Amplifier Transconductance ∆IC = ±25µA Error Amplifier Source or Sink Current VC = 1.5V Error Amplifier Clamp Voltage Hi Clamp, VFB = 1V Lo Clamp, VFB = 1.5V Reference Voltage Line Regulation 3V ≤ VIN ≤ 40V VC = 0.8V Error Amplifier Voltage Gain 0.9V ≤ VC ≤ 1.4V Minimum Input Voltage (Note 3) IQ 2 Supply Current U ABSOLUTE 3000 2400 4400 ● 6000 7000 µmho µmho 150 120 200 ● 350 400 µA µA 2.30 0.52 V V 0.03 %/V 1.80 0.25 ● 500 ● 3V ≤ VIN ≤ 40V, VC = 0.6V 0.38 800 V/V 2.6 3.0 6 9 V mA LT1572 ELECTRICAL CHARACTERISTICS SYMBOL PARAMETER Control Pin Threshold VIN = 15V, VC = 0.5V, VFB = VREF, output pin open, unless otherwise noted. CONDITIONS Duty Cycle = 0 ● Normal/Flyback Threshold on Feedback Pin VFB MIN TYP MAX UNITS 0.8 0.6 0.9 1.08 1.25 V V 0.4 0.45 0.54 V 15.0 14.0 16.3 17.6 18.0 V V 4.5 6.8 9 V 0.01 0.03 %/V 150 300 500 µmho 70 70 µA µA Flyback Reference Voltage (Note 3) IFB = 50µA Change in Flyback Reference Voltage 0.05 ≤ IFB ≤ 1mA Flyback Reference Voltage Line Regulation (Note 3) IFB = 50µA 7V ≤ VIN ≤ VMAX Flyback Amplifier Transconductance (gm) ∆IC = ±10µA Flyback Amplifier Source and Sink Current VC = 0.6V, Source IFB = 50µA, Sink ● ● 15 25 32 40 BV Output Switch Breakdown Voltage (Note 4) 3V ≤ VIN ≤ 40V, ISW = 1.5mA ● 60 80 VSAT Output Switch “On” Resistance (Note 1) ● 0.60 ● Control Voltage to Switch Current Transconductance ILIM Switch Current Limit ∆IIN ∆ISW Supply Current Increase During Switch On-Time f Switching Frequency DCMAX V 1.00 2 Duty Cycle = 50%, TJ ≥ 25°C Duty Cycle = 50%, TJ < 25°C Duty Cycle = 80% (Note 2) Maximum Switch Duty Cycle Shutdown Mode Supply Current 3V ≤ VIN ≤ 40V VC = 0.05V Shutdown Mode Threshold Voltage 3V ≤ VIN ≤ 40V ● ● ● 1.25 1.25 1.00 Ω A/V 3.0 3.5 2.5 A A A 25 35 mA/A 88 85 100 ● 112 115 kHz kHz ● 80 90 95 % 100 250 µA 150 250 300 mV mV ● 100 50 Flyback Sense Delay Time (Note 3) µs 1.5 DIODE PARAMETER CONDITIONS Forward Voltage (Note 5) If = 200mA If = 500mA If = 1A Reverse Leakage (Note 5) Diode Thermal Resistance MIN TYP MAX UNITS 0.45 0.52 0.55 0.57 0.65 0.70 V V V VR = 5V, TJ = 25°C VR = 5V, TJ = 75°C 1 25 5 100 µA µA VR = 20V, TJ = 25°C VR = 20V, TJ = 75°C 3 70 15 300 µA µA (Note 6) 90 ● ● ● °C/W 3 LT1572 ELECTRICAL CHARACTERISTICS VIN = 15V, VC = 0.5V, VFB = VREF, output pin open, unless otherwise noted. Note 5: See graphs for guaranteed forward voltage and reverse leakage current over temperature. Parameters are 100% tested at 25°C and guaranteed at other temperatures by design and QA sampling. Note 6: Package soldered to FR4 board with ≥1oz copper and an internal or backside plane underneath the package to aid thermal transfer. Diode is partly thermally coupled to regulator section. See Application Information section for details on thermal calculations. The ● denotes the specifications which apply over the full operating temperature range, 0°C to 100°C for the regulator chip and 0°C to 125°C for the diode. Note 1: Measured with VC in hi clamp, VFB = 0.8V. ISW = 1A. Note 2: For duty cycles (DC) between 50% and 80%, minimum guaranteed switch current is given by ILIM = 0.833 (2 – DC). Note 3: Minimum input voltage for isolated flyback mode is 7V. Note 4: Because the catch diode has a peak repetitive reverse voltage of 20V, diode breakdown may be the limiting factor on input voltage or switch voltage in many applications. U W TYPICAL PERFOR A CE CHARACTERISTICS Switch Current Limit vs Duty Cycle Minimum Input Voltage 4 Switch Saturation Voltage 2.9 1.6 SWITCH CURRENT (A) 3 –55°C 2 25°C 125°C 1 0 0 2.7 2.6 SWITCH CURRENT = 0A 2.5 2.4 2.3 –75 –50 –25 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) 2 1 TJ = –55°C TJ = 25°C –2 –5 10 30 40 20 INPUT VOLTAGE (V) 50 60 1572 G04 4 0.6 0.4 0.2 0 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 SWITCH CURRENT (A) 1572 G03 Feedback Bias Current vs Temperature 800 1.248 700 1.246 1.244 1.242 1.240 1.238 1.234 –75 –50 –25 600 500 400 300 200 100 1.236 0 –55°C 1.250 –3 –4 25°C 0.8 0 FEEDBACK BIAS CURRENT (nA) REFERENCE VOLTAGE (V) REFERENCE VOLTAGE CHANGE (mV) 4 TJ = 150°C 100°C 1.0 Reference Voltage vs Temperature 5 0 150°C 1.2 1572 G02 Line Regulation –1 1.4 0 25 50 75 100 125 150 TEMPERATURE (°C) 1572 G01 3 SWITCH SATURATION VOLTAGE (V) MINIMUM INPUT VOLTAGE (V) SWITCH CURRENT = 1.25A 2.8 25 50 75 100 125 150 TEMPERATURE (°C) 0 1572 G05 0 –75 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 1572 G06 LT1572 U W TYPICAL PERFOR A CE CHARACTERISTICS Supply Current vs Supply Voltage (Shutdown Mode) Driver Current* vs Switch Current 160 TJ = 25°C 140 TJ = 25°C 14 35 NOTE THAT THIS CURRENT DOES NOT INCLUDE DRIVER CURRENT, WHICH IS A FUNCTION OF LOAD CURRENT AND DUTY CYCLE. 13 100 VC = 50mV 80 60 40 30 SUPPLY CURRENT (mA) 120 DRIVER CURRENT (mA) SUPPLY CURRENT (µA) Supply Current vs Input Voltage* 15 40 25 TJ = –55°C 20 15 TJ = ≥ 25°C 10 12 11 10 90% DUTY CYCLE 9 50% DUTY CYCLE 8 10% DUTY CYCLE 7 20 5 VC = 0V 10 20 30 SUPPLY VOLTAGE (V) 0 40 1572 G07 Shutdown Mode Supply Current 4500 TRANSCONDUCTANCE (µmho) 180 140 TJ = 150°C 120 100 80 60 –55°C ≤ TJ ≤ 125°C 40 0.50 0.75 1.00 SWITCH CURRENT (A) 20 gm = 3500 3000 2500 2000 1500 6 VSUPPLY = 60V VSUPPLY = 3V 5 4 3 2 1 –75 –50 –25 0 TJ = 25°C –100 –200 VFB = 0.8V (CURRENT OUT OF VC PIN) –300 –400 0 25 50 75 100 125 150 TEMPERATURE (°C) 0 1572 G13 1.5 2.0 1.0 VC PIN VOLTAGE (V) 1000 450 900 800 400 350 300 250 2.5 Switch “Off” Characteristics 500 –55°C 25°C 150°C 200 150 700 600 500 VSUPPLY = 3V 400 VSUPPLY = 15V 300 100 200 50 100 VSUPPLY = 40V 0 0 0 25 50 75 100 125 150 TEMPERATURE (°C) 0.5 1572 G12 SWITCH CURRENT (µA) FEEDBACK VOLTAGE (mV) IDLE SUPPLY CURRENT (mA) 8 7 100 Feedback Pin Clamp Voltage 9 VFB = 1.5V (CURRENT INTO VC PIN) 200 1000 Idle Supply Current vs Temperature VC = 0.6V 1572 G09 VC Pin Characteristics 1572 G11 11 60 50 * UNDER VERY LOW OUTPUT CURRENT CONDITIONS, DUTY CYCLE FOR MOST CIRCUITS WILL APPROACH 10% OR LESS. ∆I (VC PIN) ∆V (FB PIN) 1572 G10 10 30 40 20 INPUT VOLTAGE (V) 4000 0 –75 –50 –25 10 20 30 40 50 60 70 80 90 100 VC PIN VOLTAGE (mV) 10 300 500 0 0 0 1.25 1572 G08 Error Amplifier Transconductance 5000 160 0.25 * AVERAGE POWER SUPPLY CURRENT IS FOUND BY MULTIPLYING DRIVER CURRENT BY DUTY CYCLE, THEN ADDING QUIESCENT CURRENT. 200 0% DUTY CYCLE 5 VC PIN CURRENT (µA) 0 SUPPLY CURRENT (µA) 6 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 FEEDBACK CURRENT (mA) 1572 G14 0 10 20 30 40 50 60 70 80 90 100 SWITCH VOLTAGE (V) 1572 G15 5 LT1572 U W TYPICAL PERFOR A CE CHARACTERISTICS Shutdown Thresholds – 350 250 – 250 200 – 200 VOLTAGE 150 – 150 100 –100 VC VOLTAGE IS REDUCED UNTIL REGULATOR CURRENT DROPS BELOW 300µA 0 –75 –50 –25 22 – 50 0 25 50 75 100 125 150 TEMPERATURE (°C) 0 1.8 TIME (µs) – 300 23 2.0 VC PIN CURRENT (µA) VC PIN VOLTAGE (mV) CURRENT (OUT OF VC PIN) 300 50 2.2 – 400 FLYBACK VOLTAGE (V) 400 350 1.6 1.4 1.2 1.0 –75 –50 –25 0 25 50 75 100 125 150 JUNCTION TEMPERATURE (°C) –30 90 3000 120 2000 1000 150 0 180 –1000 210 10M 1572 G19 6 FEEDBACK PIN VOLTAGE (mV) 60 gm 1M 100k FREQUENCY (Hz) 19 RFB = 1k 18 17 RFB = 10k 15 –75 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 1572 G18 500 –24 490 –22 480 –20 –18 470 460 FEEDBACK PIN VOLTAGE (AT THRESHOLD) –16 –14 450 –12 440 430 FEEDBACK PIN CURRENT (AT THRESHOLD) –10 420 –8 410 –6 400 –50 –25 0 –4 25 50 75 100 125 150 TEMPERATURE (°C) 1572 G20 FEEDBACK PIN CURRENT (µA) 30 5000 PHASE (DEG) TRANSCONDUCTANCE (µmho) 0 θ 10k 20 Normal/Flyback Mode Threshold on Feedback Pin 7000 4000 RFB = 500Ω 1572 G17 Transconductance of Error Amplifier 6000 21 16 1572 G16 1k Isolated Mode Flyback Reference Voltage Flyback Blanking Time LT1572 W BLOCK DIAGRA VIN 16V SWITCH OUT ANODE 2.3V REG FLYBACK ERROR AMP 100kHz OSC LOGIC CATHODE LT1172 5A, 75V SWITCH DRIVER ANTISAT MODE SELECT COMP – FB ERROR AMP VC + + SHUTDOWN CIRCUIT 1.24V REF 0.15V CURRENT AMP GAIN ≈ 6 0.16Ω – E1* *ALWAYS CONNECT E1 TO GROUND E2 1572 BD U OPERATIO The LT1572 is a current mode switcher. This means that switch duty cycle is directly controlled by switch current rather than by output voltage. Referring to the block diagram, the switch is turned “on” at the start of each oscillator cycle. It is turned “off” when switch current reaches a predetermined level. Control of output voltage is obtained by using the output of a voltage sensing error amplifier to set current trip level. This technique has several advantages. First, it has immediate response to input voltage variations, unlike ordinary switchers which have notoriously poor line transient response. Second, it reduces the 90° phase shift at mid-frequencies in the energy storage inductor. This greatly simplifies closedloop frequency compensation under widely varying input voltage or output load conditions. Finally, it allows simple pulse-by-pulse current limiting to provide maximum switch protection under output overload or short conditions. A low dropout internal regulator provides a 2.3V supply for all internal circuitry on the LT1572. This low dropout design allows input voltage to vary from 3V to 40V with virtually no change in device performance. A 100kHz oscillator is the basic clock for all internal timing. It turns “on” the output switch via the logic and driver circuitry. Special adaptive anti-sat circuitry detects onset of saturation in the power switch and adjusts driver current instantaneously to limit switch saturation. This minimizes driver dissipation and provides very rapid turn-off of the switch. A 1.2V bandgap reference biases the positive input of the error amplifier. The negative input is brought out for output voltage sensing. This feedback pin has a second function; when pulled low with an external resistor, it programs the LT1572 to disconnect the main error amplifier output and connects the output of the flyback amplifier 7 LT1572 U OPERATIO to the comparator input. The LT1572 will then regulate the value of the flyback pulse with respect to the supply voltage.1 This flyback pulse is directly proportional to output voltage in the traditional transformer coupled flyback topology regulator. By regulating the amplitude of the flyback pulse, the output voltage can be regulated with no direct connection between input and output. The output is fully floating up to the breakdown voltage of the transformer windings. Multiple floating outputs are easily obtained with additional windings. A special delay network inside the LT1572 ignores the leakage inductance spike at the leading edge of the flyback pulse to improve output regulation. Other Application Help The error signal developed at the comparator input is brought out externally. This pin (VC) has four different functions. It is used for frequency compensation, current limit adjustment, soft starting, and total regulator shutdown. During normal regulator operation this pin sits at a voltage between 0.9V (low output current) and 2.0V (high output current). The error amplifiers are current output (gm) types, so this voltage can be externally clamped for adjusting current limit. Likewise, a capacitor coupled external clamp will provide soft start. Switch duty cycle goes to zero if the VC pin is pulled to ground through a diode, placing the LT1572 in an idle mode. Pulling the VC pin below 0.15V causes total regulator shutdown, with only 50µA supply current for shutdown circuitry biasing. See AN19 for full application details. Thermal Management More circuits and application help for the LT1572 can be found in the LT1172 data sheet, both in loose form and in the 1994 Linear Databook Volume III. Extensive additional help is contained in Application Note 19. All application circuits using the LT1172 can also use the LT1572 as long as the 20V maximum reverse voltage of the diode is not exceeded. A CAD program called SwitcherCAD is also available. This program can be used with the LT1572 by simply treating the LT1572 as an LT1172 and ignoring the predicted die temperature results obtained from SwitcherCAD itself. Thermal management is particularly important with the LT1572 because both switch and diode power dissipation increase rapidly at low input voltage when using the popular boost topology. Regulator and diode die temperature must be calculated separately because they are not connected to an isothermal plane inside the package. Diode plus regulator thermal resistance is approximately 70°C/W when the LT1572 is soldered to 1oz copper traces over an internal or backside copper plane using FR4 board material. However, individual calculation of die temperature must take thermal coupling into account. To accomplish this, thermal resistance is broken into two sections, a common (coupled) section and a second uncoupled section. Die temperatures are calculated from: E1 and E2 Pins TREG = TA + PREG (90°C/W) + PDIODE (45°C/W) The LT1572 has the emitters of the power transistor brought out separately from the ground pin. This eliminates errors due to ground pin voltage drops and allows the user to reduce switch current limit 2:1 by leaving the second emitter (E2) disconnected. The first emitter (E1) should always be connected to the ground pin. Note that switch “on” resistance doubles when E2 is left open, so efficiency will suffer somewhat when switch currents exceed 300mA. Also, note that chip dissipation will actually increase with E2 open during normal load operation, even though dissipation in current limit mode will decrease. TDIODE = TA + PDIODE (90°C/W) + PREG (45°C/W) 1See note under block diagram. 8 TA = ambient temperature TREG = regulator die temperature TDIODE = diode die temperature PREG = total regulator power dissipation PDIODE = diode power dissipation The following formulas can be used as a rough guide to calculate LT1572 power dissipation. For more details, the reader is referred to Application Note 19 (AN19), “Efficiency Calculations” section. LT1572 U OPERATIO Average supply current (including driver current) is: IIN ≈ 6mA + ISW (0.004 + DC/40) ISW = switch current DC = switch duty cycle Switch power dissipation is given by: PSW = (ISW)2 × RSW × DC RSW = LT1572 switch “on” resistance (1Ω maximum) Total power dissipation is the sum of supply current times input voltage plus switch power: PREG = IIN × VIN + PSW In a typical example, using a boost converter to generate 12V at 0.12A from a 5V input, duty cycle is approximately 60%, and switch current is about 0.65A, yielding: IIN = 6mA + 0.65(0.004 + DC/40) = 18mA PSW = (0.65)2 × 1Ω × 0.6 = 0.25W PREG = 5V × 0.018A + 0.25 = 0.34W Approximate diode power dissipation for boost and buck converters is shown below. For other topologies or more accurate results, see Application Note 19 or use SwitcherCAD. Boost: PDIODE = IOUT × Vf Buck: PDIODE = IOUT × Vf × (VIN – VOUT)/VIN Vf = diode forward voltage at a current equal to IOUT for a buck converter and IOUT × VOUT/VIN for a boost converter. In most applications, full load current is used to calculate die temperature. However, if overload conditions must also be accounted for, three approaches are possible. First, if loss of regulated output is acceptable under overload conditions, the internal thermal limit of the LT1572 will protect the die in most applications by shutting off switch current. Thermal limit is not a tested parameter, however, and should be considered only for noncritical applications with temporary overloads. The second approach for lower current applications is to leave the second switch emitter (E2) open. This increases switch “on” resistance by 2:1, but reduces switch current limit by 2:1 also, resulting in a net 2:1 reduction in I2R switch dissipation under current limit conditions. The third approach is to clamp the VC pin to a voltage less than its internal clamp level of 2V. The LT1172 switch current limit is zero at approximately 1V on the VC pin and 2A at 2V on the VC pin. Peak switch current can be externally clamped between these two levels with a diode. See AN19 for details. Diode Characteristics The catch diode used in the LT1572 is a power Schottky diode with a very low storage time and low forward voltage. This gives good efficiency in switching regulator applications, but some thought must be given to maximum operating voltage and high temperature reverse leakage. Peak repetitive reverse voltage rating on the diode is 20V. In a boost converter, maximum diode reverse voltage is equal to regulated output voltage, so this limits maximum output voltage to 20V. In a negative-to-positive converter, maximum diode voltage will be equal to the sum of output voltage plus input voltage. Use the equations in Application Note 19 or SwitcherCAD or calculate maximum diode voltage for other topologies. Diode reverse leakage increases rapidly with temperature. This leakage is not high enough to significantly impact efficiency or diode power dissipation, but it can be of concern in shutdown mode if the diode is connected in such a way that the leakage adds to regulator shutdown current. Use the graphs of diode leakage versus voltage and temperature to ensure proper high temperature system performance. The LT1572 diode is internally bonded to more than two package pins to reduce internal bond wire currents. All pins must be used to prevent excessive current in the individual internal bond wires. This is important in low load current applications because the LT1572 will draw high surge currents during start-up (to charge the output capacitor) even with no output load current. 9 LT1572 U OPERATIO Synchronizing with Bipolar Transistor Synchronizing The LT1572 can be externally synchronized in the frequency range of 120kHz to 160kHz. This is accomplished as shown in the accompanying figures. Synchronizing occurs when the VC pin is pulled to ground with an external transistor. To avoid disturbing the DC characteristics of the internal error amplifier, the width of the synchronizing pulse should be under 0.3µs. C2 sets the pulse width at ≅ 0.2µs. The effect of a synchronizing pulse on the LT1572 amplifier offset can be calculated from: VIN LT1572 VC GND C2 39pF R3 C1 KT VC q tS fS IC + R3 ∆VOS = IC ( )( ) KT = 26mV at 25°C q tS = pulse width fS = pulse frequency IC = VC source current (≈ 200µA) VC = operating VC voltage (1V to 2V) R3 = resistor used to set mid-frequency “zero” in frequency compensation network. With tS = 0.2µs, fS = 150kHz, VC = 1.5V, and R3 = 2k, offset voltage shift is ≈ 3.8mV. This is not particularly bothersome, but note that high offsets could result if R3 were reduced to a much lower value. Also, the synchronizing transistor must sink higher currents with low values of R3, so larger drives may have to be used. The transistor must be capable of pulling the VC pin to within 200mV of ground to ensure synchronizing. 10 R1 3k 2N2369 R2 2.2k FROM 5V LOGIC 1572 OP01 Synchronizing with MOS Transistor VIN LT1572 GND VC R3 C1 D1 1N4158 C2 100pF VN2222* R2 2.2k D2 1N4158 FROM 5V LOGIC *SILICONIX OR EQUIVALENT 1572 OP02 LT1572 UO TYPICAL APPLICATI S Negative Buck Converter + * REQUIRED IF INPUT LEADS ≥ 2" ** PULSE ENGINEERING 92114 COILTRONICS 50-2-52 CATHODE L1** 50µH VIN C2 200µF VSW + C3* 100µF E1 Q1 2N3906 LT1572 ANODE E2 LOAD R1 4.64k R4 12k –5.2V 0.75A FB VC GND C1 R2 1.24k R3 VIN –7V TO –20V 1572 TA03 Backlight CCFL Supply (see AN55 for details) INPUT VOLTAGE† 4.5V TO 20V L2*** 1k L1** 300µH A 33pF 3kV LAMP Q1* CATHODE 10µF TANT + VIN VSW E2 0.02µF B GND VC + 2µF D2 1N914 Q2* LT1572 ANODE E1 D1 1N914 R3 10k 50k INTENSITY ADJUST R1 560Ω FB C6 1µF * Q1,Q2 = BCP56 OR MPS650/561 ** COILTRONICS CTX300-4 *** SUMIDA 6345-020 OR COILTRONICS 110092-1 † A MODIFICATION WILL ALLOW OPERATION DOWN TO 4.5V. CONSULT FACTORY. Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 1572 TA04 11 LT1572 U PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. S Package 16-Lead Plastic SOIC 0.386 – 0.394* (9.804 – 10.008) 16 15 14 13 12 11 10 9 0.150 – 0.157* (3.810 – 3.988) 0.228 – 0.244 (5.791 – 6.197) 1 0.010 – 0.020 × 45° (0.254 – 0.508) 0.008 – 0.010 (0.203 – 0.254) 2 3 5 4 6 7 0.053 – 0.069 (1.346 – 1.752) 0.004 – 0.010 (0.101 – 0.254) 0° – 8° TYP 0.016 – 0.050 0.406 – 1.270 8 0.050 (1.270) TYP 0.014 – 0.019 (0.355 – 0.483) SO16 0893 *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm). RELATED PARTS PART NUMBER DESCRIPTION LT1172 100kHz, 1.25A High Efficiency Switching Regulator LT1572 Without Diode LT1173 Micropower DC/DC Converter Adjustable and Fixed 5V, 12V Operates Down to 2V Input LT1372 500kHz High Efficiency 1.5A Step-Up Switching Regulator Latest Technology, Uses Tiny Inductors LTC1574 High Efficiency Step-Down DC/DC Converter with Internal Schottky Diode LTC1174 with Diode 12 Linear Technology Corporation COMMENTS LT/GP 0595 10K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7487 (408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977 LINEAR TECHNOLOGY CORPORATION 1995