LT1082CT

LT1082 1A High Voltage, Efficiency Switching Voltage Regulator FEATURES s s s s s s DESCRIPTIO s s s Wide Input Voltage Range: 3V to 75V High Switch Voltage: 100V Low Quiescent Current: 4.5mA Internal 1A Switch Shutdown Mode Draws Only 120µA Supply Current Isolated Flyback Regulation Mode for Fully Floating Outputs Can Be Externally Synchronized Available in MiniDIP and TO-220 Packages Same Pinout as LT1072 The LT1082 is a monolithic high voltage switching regulator. It can be operated in all standard switching configurations including buck, boost, flyback, forward, and inverting. A 1A high efficiency switch is included on the die along with all oscillator, control, and protection circuitry. The LT1082 operates with supply voltages from 3V to 75V, switch voltage up to 100V and draws only 4.5mA quiescent current. It can deliver load power up to 20W with no external power devices. By utilizing current-mode switching techniques, it provides excellent AC and DC load and line regulation. An externally activated shutdown mode reduces total supply current to 120µA typical for standby operation. Totally isolated and regulated outputs can be generated by using the optional “isolated flyback regulation mode” built into the LT1082, without the need for optocouplers or extra transformer windings. The LT1082 has a unique feature to provide high voltage short-circuit protection. When the FB pin is pulled down to 0.6V and the current out of the pin reaches approximately 350µA, the switching frequency will shift down from 60kHz to 12kHz. The LT1082 is nearly identical to the lower voltage LT1072. For the major differences in specifications, see the table on the left. Telecom 5V Supply Maximum Output Current vs Input Voltage MAXIMUM OUTPUT CURRENT (A) APPLICATI s s s S Telecom 5V Supply at 0.7A from –48V 90V Supply at 120mA from 15V All Applications Using LT1072 (See Below for Specification Differences) LT1082 and LT1072 Major Specification Differences LT1082C LT1072HV 3V to 75V 3V to 60V 100V 75V 1A 1.25A 4.5mA 6mA 60kHz 40kHz 16.2 + 0.6 (35kΩ /RFB) 16 + 0.35 (7kΩ /RFB) VIN VSW Switch Current Limit Quiescent Current Operating Frequency Flyback Reference Voltage USER NOTE: This data sheet is only intended to provide specifications, graphs, and a general functional description of the LT1082. Application circuits are included to show the capability of the LT1082. A complete design manual (AN19) and Switcher CAD (LTC Switching Power Supply Design Program) should be obtained to assist in developing new designs. This manual contains a comprehensive discussion of both the LT1070 and the external components used with it, as well as complete formulas for calculating the values of these components. The manual can also be used for the LT1082 by factoring in the lower switch current rating. Negative-to-Positive Telecom 5V Supply **250µH *D1 3.83k VIN 33µF 80V CHEMI-CON SXE SERIES VSW Q1 2N5401 + 470µF 10V CHEMI-CON SXE SERIES VOUT 5V, 0.7A NOTE: MAXIMUM OUTPUT CURRENT IS A FUNCTION OF INPUT VOLTAGE. SEE THE GRAPH ON THE RIGHT. + GND LT1082 VC FB 1.1k 4.7k 0.22µF 0.01µF * MOTOROLA MUR110 (100V, 1A) ** 69 TURNS OF #28 AWG WIRE ON A MICROMETALS T60 TYPE 52 CORE. NOTE: THIS CORE IS LOW COST, BUT HAS HIGHER CORE LOSS AND IS LARGER THAN NECESSARY FOR LOWER CURRENT APPLICATIONS. FOR SMALLER INDUCTORS OR HIGHER EFFICIENCY, USE A LOW LOSS CORE SUCH AS MAGNETICS INC. KOOL Mµ OR MOLYPERMALLOY. –20V TO –70V 1082 TA01 U 1.0 0.9 0.8 0.7 0.6 0.5 L=100µH 0.4 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 INPUT VOLTAGE (V) 1082 TA02 UO f = 45kHz ISW LIMIT = 1.07A L=550µH L=450µH L=350µH L=250µH L=150µH 1 LT1082 ABSOLUTE AXI U RATI GS Operating Junction Temperature Range LT1082M ......................................... – 55°C to 150°C LT1082I ........................................... – 40°C to 125°C LT1082C ............................................... 0°C to 100°C Supply Voltage ....................................................... 75V Switch Output Voltage .......................................... 100V Feedback Pin Voltage (Transient, 1ms) ................ ±15V Storage Temperature Range ................ – 65°C to 150°C Lead Temperature (Soldering, 10 sec)................. 300°C PACKAGE/ORDER I FOR ATIO TOP VIEW GND 1 VC 2 FB 3 NC 4 8 7 6 5 E2 VSW E1 VIN FRONT VIEW 5 4 3 2 1 Q PACKAGE 5-LEAD DD TJMAX = 100°C, θJA = 40°C/ W (CQ) TJMAX = 125°C, θJA = 40°C/ W (IQ) NOTE: θJA VARIES FROM 25°C/W TO 50°C/W DEPENDING ON BOARD COMPOSITION. VIN VSW GND FB VC J8 PACKAGE N8 PACKAGE 8-LEAD CERAMIC DIP 8-LEAD PLASTIC DIP TJMAX = 150°C, θJA = 100°C/W (MJ8) TJMAX = 100°C, θJA = 90°C/W (CN8) TJMAX = 125°C, θJA = 90°C/W (IN8) ORDER PART NUMBER LT1082MJ8 LT1082CN8 LT1082IN8 ORDER PART NUMBER LT1082CQ LT1082IQ ELECTRICAL CHARACTERISTICS VIN = 15V, VC = 0.5V, VFB = VREF, output pin open, unless otherwise specified. SYMBOL VREF IB gm PARAMETER Reference Voltage Feedback Input Current Error Amplifier Transconductance Error Amplifier Source or Sink Current Error Amplifier Clamp Voltage Reference Voltage Line Regulation AV Error Amplifier Voltage Gain Minimum Input Voltage CONDITIONS Measured at Feedback Pin VC = 0.8V VFB = VREF q q ∆IC = ±25µA q VC = 1.5V q Hi Clamp, VFB = 1V Lo Clamp, VFB = 1.5V 3V ≤ VIN ≤ VMAX, VC = 0.8V 0.9V ≤ VC ≤ 1.4V q q 2 U U W WW U W FRONT VIEW 5 4 3 2 1 T PACKAGE 5-LEAD TO-220 VIN VSW GND FB VC TJMAX = 100°C, θJA = 75°C/ W, θJC = 8°C/ W (CT) TJMAX = 125°C, θJA = 75°C/ W, θJC = 8°C/ W (IT) ORDER PART NUMBER LT1082CT LT1082IT MIN TYP MAX 1.264 1.274 750 1100 6000 7000 400 400 2.3 0.36 0.03 UNITS V V nA nA µmho µmho µA µA V V %/V V/V V 1.224 1.244 1.214 1.244 350 3000 2400 150 120 1.8 0.12 350 4400 200 0.22 650 2.6 3.0 LT1082 ELECTRICAL CHARACTERISTICS VIN = 15V, VC = 0.5V, VFB = VREF, output pin open, unless otherwise specified. SYMBOL IQ PARAMETER Supply Current Control Pin Threshold Normal/Flyback Threshold on Feedback Pin f Switching Frequency q CONDITIONS 3V ≤ VIN ≤ VMAX, VC = 0.6V Duty Cycle = 0 q MIN TYP 4.5 MAX 7.0 1.1 1.25 0.8 70 75 UNITS mA V V V kHz kHz kHz V A/V 0.7 0.5 0.58 50 45 100 0.9 0.67 60 12 800µA ≥ IFB ≥ 450µA BV Output Switch Breakdown Voltage Control Voltage to Switch Current Transconductance VFB Flyback Reference Voltage Change in Flyback Reference Voltage Flyback Reference Voltage Line Regulation Flyback Amplifier Transconductance (gm) Flyback Amplifier Source and Sink Current VSAT ILIM Output Switch “On” Resistance (Note 1) Switch Current Limit (LT1082C) Switch Current Limit (LT1082I) Switch Current Limit (LT1082M) ∆IIN ∆ISW DCMAX Supply Current Increase During Switch-On Time Maximum Switch Duty Cycle Flyback Sense Delay Time Shutdown Mode Supply Current Shutdown Mode Threshold Voltage 3V ≤ VIN ≤ VMAX, VC = 0.05V 3V ≤ VIN ≤ VMAX q 3V ≤ VIN ≤ VMAX, ISW = 1.5mA q 115 1.5 IFB = 60µA q 17 16 3.5 150 18.6 4.6 0.01 300 32 50 0.8 20.5 21.5 6.5 0.03 500 70 90 1.2 2.6 2.6 2.4 2.8 2.8 2.6 3.0 3.0 2.8 45 97 350 250 300 V V V %/V µmho µA µA Ω A A A A A A A A A mA/A % µs µA mV mV 60µA ≤ IFB ≤ 200µA IFB = 60µA, 3V ≤ VIN ≤ VMAX ∆IC = ±10µA VC = 0.6V Source IFB = 60µA Sink ISW = 0.7A (LT1082C), ISW = 0.5A (LT1082M) Duty Cycle = 20% Duty Cycle ≤ 50% Duty Cycle = 80% (Note 2) Duty Cycle = 20% Duty Cycle ≤ 50% Duty Cycle = 80% (Note 2) Duty Cycle = 20% Duty Cycle ≤ 50% Duty Cycle = 80% (Note 2) q q q q q q q q q q q q 15 30 1.07 1.0 0.8 0.85 0.8 0.65 0.75 0.7 0.6 35 85 92 1.5 120 150 70 50 The q denotes the specifications which apply over the operating temperature range. Note 1: Measured with VC in hi clamp, VFB = 0.8V. Note 2: For duty cycles (DC) between 50% and 80%, minimum guaranteed switch current decreases linearly. 3 LT1082 TYPICAL PERFOR A CE CHARACTERISTICS Suggested Core Size and Inductance for Telecom 5V Supply LOAD CURRENT 100mA 200mA 400mA 600mA 800mA TYPE 52 POWDERED IRON T38 250µH T50 250µH T60 250µH T60 250µH T80 350µH KOOL Mµ OR MOLYPERMALLOY T38 200µH T38 150µH T50 150µH T50 200µH T80 350µH 1082 GA 70 67 64 61 58 55 0 VIN = – 40V FREQUENCY (kHz) EFFICIENCY (%) Short-Circuit Frequency Shift-Down vs Feedback Current 70 60 FREQUENCY (kHz) TA = 0°C TA = 150°C TA = – 55°C SWITCH CURRENT (A) DUTY CYCLE (%) 50 40 30 20 10 0 0 100 200 300 400 500 600 700 800 FEEDBACK CURRENT (µA) 1082 G03 Flyback Blanking Time 2.2 2.0 1.8 2.9 SWITCH SATURATION VOLTAGE (V) MINIMUM INPUT VOLTAGE (V) TIME (µs) 1.6 1.4 1.2 1.0 –75 –50 –25 0 25 50 75 100 125 150 JUNCTION TEMPERATURE (°C) 1082 G06 4 UW Telecom 5V Supply Efficiency 79 76 73 VIN = – 20V 80 70 60 50 40 30 20 Telecom 5V Supply Short-Circuit Frequency Shift-Down TA = 25°C VIN = – 60V VIN = – 70V L = 250µH R = 0.08Ω 2 3 4 10 0 6 5 4 3 2 OUTPUT VOLTAGE (V) 1 0 1082 G02 1 POWER OUTPUT (W) NOTE: THIS GRAPH IS BASED ON LOW CORE LOSS PERMALLOY INDUCTOR. IF POWDERED IRON CORE INDUCTOR IS USED, THE CORE LOSS IS TYPICALLY 100mW HIGHER. 1082 G01 Switch Current Limit 4 Maximum Duty Cycle 97 96 3 95 94 93 92 91 2 TJ = – 55°C TJ = 150°C TJ = 25°C 1 0 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) 1082 G04 90 –75 –50 –25 0 25 50 75 100 125 150 175 TEMPERATURE (°C) 1082 G05 Minimum Input Voltage 2.00 1.75 Switch Saturation Voltage 2.8 2.7 ISW = 1A TJ = 150°C 1.50 1.25 1.00 0.75 0.50 0.25 0 0 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 SWITCH CURRENT (A) 1082 G08 TJ = 100°C TJ = 25°C ISW = 0A 2.6 2.5 2.4 2.3 –75 –50 –25 TJ = – 50°C 0 25 50 75 100 125 150 TEMPERATURE (°C) 1082 G07 LT1082 TYPICAL PERFOR A CE CHARACTERISTICS Isolated Mode Flyback Reference Voltage 25 24 23 FLYBACK VOLTAGE (V) 65 FREQUENCY (kHz) VREF FREQ 1.245 REFERENCE VOLTAGE CHANGE (mV) RFB = 3k 22 21 20 19 18 17 16 15 –75 –50 –25 0 25 50 75 100 125 150 175 TEMPERATURE (°C) 1082 G09 RFB = 6k RFB = 10k Feedback Bias Current vs Temperature 800 700 FEEDBACK BIAS CURRENT (nA) 675 650 625 600 575 FEEDBACK PIN CURRENT (AT THRESHOLD) 550 525 –18 –16 –14 –12 –10 –8 –6 SUPPLY CURRENT (µA) VC PIN VOLTAGE (mV) 600 500 400 300 200 100 0 –75 –50 –25 0 25 50 75 100 125 150 175 TEMPERATURE (°C) 1082 G12 Supply Current vs Supply Voltage (Shutdown Mode) 200 160 DRIVER CURRENT (mA) 120 VC = 50mV 70 60 50 40 30 20 10 SUPPLY CURRENT (mA) SUPPLY CURRENT (µA) 80 VC = 0V 40 0 0 10 50 60 20 30 40 SUPPLY VOLTAGE (V) 70 80 UW 1082 G15 Reference Voltage and Switching Frequency vs Temperature 70 1.250 5 4 3 2 1 0 –1 –2 –3 –4 –5 Line Regulation REFERENCE VOLTAGE (V) 60 1.240 TJ = – 55°C 55 1.235 50 1.230 TJ = 150°C TJ = 25°C 45 1.225 –75 –50 –25 0 25 50 75 100 125 150 175 TEMPERATURE (°C) 1082 G11 0 10 20 30 40 50 60 INPUT VOLTAGE (V) 70 80 Normal/Feedback Mode Threshold on Feedback Pin 750 725 FEEDBACK PIN VOLTAGE (AT THRESHOLD) 700 –24 –22 –20 VC PIN CURRENT (µA) 200 180 160 140 120 100 80 60 40 20 0 1082 G10 Shutdown Mode Supply Current –55°C ≤ TJ ≤ 125°C TJ = 150°C 500 –4 –75 –50 –25 0 25 50 75 100 125 150 175 TEMPERATURE (°C) 1082 G16 0 10 20 30 40 50 60 70 80 90 100 VC PIN VOLTAGE (mV) 1082 G17 Driver Current* vs Switch Current 100 90 80 Supply Current vs Input Voltage** 14 13 12 11 10 9 8 7 6 5 4 3 2 0% DUTY CYCLE 50% DUTY CYCLE 10% DUTY CYCLE 90% DUTY CYCLE 0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 SWITCH CURRENT (A) 0 10 20 30 40 50 60 INPUT VOLTAGE (V) 70 80 * AVERAGE SUPPLY CURRENT = I Q + DC(2.9 + 10 –2 ISW + 10 –5 ISW2) IQ = QUIESCENT CURRENT, DC = DUTY CYCLE, 1082 G13 ISW = SWITCH CURRENT **UNDER VERY LOW OUTPUT CURRENT CONDITIONS, DUTY CYCLE FOR MOST CIRCUITS WILL APPROACH 1082 G14 10% OR LESS. 5 LT1082 TYPICAL PERFOR A CE CHARACTERISTICS Error Amplifier Transconductance 5000 4500 TRANSCONDUCTANCE (µmho) gm = ∆I (VC PIN) ∆V (FB PIN) IDLE SUPPLY CURRENT (mA) 4000 300 VC VOLTAGE (mV) 3500 3000 2500 2000 1500 1000 500 0 –75 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 1082 G18 Feedback Pin Clamp Voltage 800 700 500 450 FEEDBACK VOLTAGE (mV) SWITCH CURRENT (µA) 600 TJ = 25°C 500 400 300 200 TJ = 150°C 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 FEEDBACK CURRENT (mA) 1082 G21 VC Pin Characteristics 400 300 TJ = 25°C TRANSCONDUCTANCE (µmho) VC PIN CURRENT (µA) 200 100 0 –100 –200 –300 –400 0 VFB = 0.8V (CURRENT OUT OF VC PIN) 0.5 6 UW Shutdown Thresholds 400 350 CURRENT (OUT OF VC PIN) –400 –350 –300 VC CURRENT (µA) Idle Supply Current vs Temperature 10 9 8 7 6 5 4 3 2 1 –75 –50 –25 0 25 50 75 100 125 150 175 TEMPERATURE (°C) 1082 G20 VC = 0.6V 250 200 VOLTAGE AT VC PIN 150 100 50 –250 –200 –150 –100 –50 VIN = 75V VIN = 3V 0 0 –75 –50 –25 0 25 50 75 100 125 150 175 TEMPERATURE (°C) 1082 G19 Switch “Off” Characteristics A. VIN = 3V B. VIN = 15V C. VIN = 40V D. VIN = 55V E. VIN = 75V TJ = – 55°C 400 350 300 250 200 150 100 50 0 0 A B C D E 10 20 30 40 50 60 70 80 90 100 SWITCH VOLTAGE (V) 1082 G22 Transconductance of Error Amplifier 7000 6000 5000 4000 3000 2000 1000 0 –1000 1k 10k 100k 1M FREQUENCY (Hz) gm θ –30 0 30 60 90 120 150 180 210 10M 1082 G24 VFB = 1.5V (CURRENT INTO VC PIN) PHASE (DEG) 1.5 2.0 1.0 V C PIN VOLTAGE (V) 2.5 1082 G23 LT1082 BLOCK DIAGRA FB 1.24V REF OPERATIO The LT1082 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 + – W VIN 16.2V SWITCH OUT 2.3V REG FLYBACK ERROR AMP OSC 60kHz 14kHz LOGIC DRIVER MODE SELECT COMP ANTI-SAT + VC CURRENT AMP GAIN ≈ 5 SHUTDOWN CIRCUIT 0.2Ω 0.2Ω ERROR AMP – E1* E2 0.15V * ALWAYS CONNECT E1 TO GROUND PIN ON MiniDIP PACKAGE. EMITTERS TIED TO GROUND ON TO-220 PACKAGE. GND 1082 BD U low dropout internal regulator provides a 2.3V supply for all internal circuitry on the LT1082. This low dropout design allows input voltage to vary from 3V to 75V with virtually no change in device performance. A 60kHz 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 and with IFB of 60µA to 200µA, it programs the LT1082 to 7 LT1082 OPERATIO disconnect the main error amplifier output and connects the output of the flyback amplifier to the comparator input. The LT1082 will then regulate the value of the flyback pulse with respect to the supply voltage. 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 LT1082 ignores the leakage inductance spike at the leading edge of the flyback pulse to improve output regulation. When IFB drawn out of the FB pin reaches 350µA, the LT1082 shifts the switching frequency down to 12kHz. This unique feature provides high voltage short-circuit protection in systems like the telecom 5V supplies with input voltages down to – 70V; lower frequency is needed under short-circuit conditions with current mode switchers because minimum “on” time cannot be forced below the internally set blanking time. Referring to the telecom 5V supply circuit on the front page, with output shorted to ground, the VFB stays at 0.6V when sourcing IFB up to 1mA. If the FB pin is forced to source more than 1mA, the frequency shifting function may be defeated. Therefore, the minimum suggested value for RFB is 1k and the maximum suggested value is 1.2k. Also, no capacitance more than 1nF should be used on the FB pin, because it may cause unstable switching frequency in this low frequency mode. 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 2V (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 LT1082 in an idle mode. Pulling the VC pin below 0.15V causes total regulator shutdown, with 8 U only 120µA supply current for shutdown circuitry biasing. See AN19 for full application details. Extra Pins on the MiniDIP Packages The miniDIP LT1082 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 by a factor of 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 100mA. Also, note that chip dissipation will actually increase with E2 open during normal load operation, even though dissipation in current limit mode will decrease. See “Thermal Considerations.” Thermal Considerations When Using the MiniDIP Packages The low supply current and high switch efficiency of the LT1082 allow it to be used without a heat sink in most applications when the TO-220 package is selected. This package is rated at 50°C/W. The miniDIPs, however, are rated at 100°C/W in ceramic (J) and 90°/W in plastic (N). Care should be taken for miniDIP applications to ensure that the worst case input voltage and load current conditions do not cause excessive die temperatures. The following formulas can be used as a rough guide to calculate LT1082 power dissipation. For more details, the reader is referred to Application Note 19 (AN19), “Efficiency Calculations” section. Average supply current (including driver current) is: IIN ≈ 4.5mA + ISW (0.004 + DC/28) ISW = switch current DC = switch duty cycle Switch power dissipation is given by: PSW = (ISW)2 • RSW • DC RSW = LT1082 switch “on” resistance (1.2Ω maximum) LT1082 OPERATIO Total power dissipation is the sum of supply current times input voltage plus switch power: PTOT = (IIN)(VIN) + PSW In a typical example, using negative-to-positive converter to generate 5V at 0.5A from a – 45V input, duty cycle is approximately 12%, and switch current is about 0.5A, yielding: IIN = 4.5mA + 0.5(0.004 + DC/28) = 8.7mA PSW = (0.5)2 • 1.2Ω • (0.12) = 0.036W PTOT = (45V)(8.7mA) + 0.036 = 0.43W Temperature rise in a plastic miniDIP would be 90°C/W times 0.43W, or approximately 39°C. The maximum ambient temperature would be limited to 100°C (commercial temperature limit) minus 39°C, or 61°C. In most applications, full load current is used to calculate die temperature. However, if overload conditions must also be accounted for, four approaches are possible. First, if loss of regulated output is acceptable under overload conditions, the internal thermal limit of the LT1082 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. A second approach is to use the larger TO-220 (T) package which, even without a heat sink, may limit die temperatures to safe levels under overload conditions. In critical situations, heat sinking of these packages is required; especially if overload conditions must be tolerated for extended periods of time. The third approach for lower current applications is to leave the second switch emitter (miniDIP only) 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 fourth approach is to clamp the VC pin to a voltage less than its internal clamp level of 2V. The LT1082 switch current limit is zero at approximately 1V on the VC pin and 1.6A at 2V on the VC pin. Peak switch current can be externally clamped between these two levels with a diode. See AN19 for details. U LT1082 Synchronizing The LT1082 can be externally synchronized in the frequency range of 75kHz to 90kHz. 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 1µs. C2 sets the pulse width at ≈ 0.6µs. The effect of a synchronizing pulse on the LT1082 amplifier offset can be calculated from:  KT  VC     ( tS )( fS) IC +   q R3  ∆VOS = IC KT/q = 26mV at 25°C tS = pulse width fS = pulse frequency IC = LT1082 VC source current (≈ 200µA) VC = LT1082 operating VC voltage (1V to 2V) R3 = resistor used to set mid-frequency “zero” in LT1082 frequency compensation network. With tS = 0.6µs, fS = 80kHz, VC = 1.5V, and R3 = 2k, offset voltage shift is ≈ 5mV. This is not particularly bothersome, but note that high offset 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 100mV of ground to ensure synchronizing. Synchronizing the LT1082 VIN LT1082 GND VC C2 350pF R3 VN2222* C1 D1 1N4148 R2 2.2k D2 1N4148 FROM 5V LOGIC *SILICONIX OR EQUIVALENT 1082 OP01 9 LT1082 TYPICAL APPLICATI VIN 30V to 70V VIN 15V 25µF 10 UO + + S Totally Isolated Converter 1.24:1 25V LPRI 500µH 1 1 MUR110 15V AT 0.3A 200µF COM + + MUR110 VIN 25µF LT1082 FB GND VC 3k 0.01µF 7k VSW 200µF –15V AT 0.3A MUR110 MINIMUM LOAD OF 0.15A IS REQUIRED FOR EACH OUTPUT. (SEE AN19) 1082 TA03 Boost Converter 1mH, 1A MUR110 VOUT 90V AT 120mA 78.8k VIN LT1082 VSW FB + GND VC 0.033µF 100µF 4.7k 1.1k 0.22µF 1082 TA04 LT1082 PACKAGE DESCRIPTIO 0.290 – 0.320 (7.366 – 8.128) CORNER LEADS OPTION (4 PLCS) 0.008 – 0.018 (0.203 – 0.460) 0.385 ± 0.025 (9.779 ± 0.635) 0° – 15° 0.023 – 0.045 (0.58 – 1.14) HALF LEAD OPTION 0.045 – 0.065 (1.14 – 1.65) FULL LEAD OPTION 0.045 – 0.065 (1.14 – 1.65) 0.014 – 0.026 (0.360 – 0.660) 0.300 – 0.320 (7.620 – 8.128) 0.009 – 0.015 (0.229 – 0.381) 0.065 (1.651) TYP 0.125 (3.175) MIN 0.020 (0.508) MIN ( +0.025 0.325 –0.015 +0.635 8.255 –0.381 ) 0.045 ± 0.015 (1.143 ± 0.381) 0.100 ± 0.010 (2.540 ± 0.254) 0.060 (1.524) ( +0.012 0.331 –0.020 +0.305 8.407 –0.508 ) 0.067 ± 0.010 (1.702 ± 0.254) 0.032 ± 0.008 (0.813 ± 0.203) ( +0.012 0.143 –0.020 +0.305 3.632 –0.508 ) 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. U Dimensions in inches (milimeters) unless otherwise noted. J8 Package 8-Lead Ceramic DIP 0.200 (5.080) MAX 0.015 – 0.060 (0.381 – 1.524) 0.005 (0.127) MIN 0.405 (10.287) MAX 8 7 6 5 0.025 (0.635) RAD TYP 0.125 3.175 0.100 ± 0.010 MIN (2.540 ± 0.254) 1 2 3 0.220 – 0.310 (5.588 – 7.874) 4 N8 Package 8-Lead Plastic DIP 0.045 – 0.065 (1.143 – 1.651) 0.130 ± 0.0 (3.302 ± 0.127) 0.400 (10.160) MAX 8 7 6 5 0.250 ± 0.010 (6.350 ± 0.254) 1 2 3 4 0.018 ± 0.003 (0.457 ± 0.076) Q Package 5-Lead Plastic DD 0.401 ± 0.015 (10.185 ± 0.381) 15° TYP 0.175 ± 0.008 (4.445 ± 0.203) 0.050 ± 0.008 (1.270 ± 0.203) +0.008 0.004 –0.004 0.059 (1.499) TYP ( +0.203 0.102 –0.102 ) 0.105 ± 0.008 (2.667 ± 0.203) 0.022 ± 0.005 (0.559 ± 0.127) 0.050 ± 0.012 (1.270 ± 0.305) 11 LT1082 PACKAGE DESCRIPTIO 0.380 – 0.420 (9.652 – 10.668) 0.139 – 0.153 (3.531 – 3.886) DIA 0.560 – 0.650 (14.224 – 16.510) 0.866 – 0.913 (21.996 – 23.190) 0.057 – 0.077 (1.448 – 1.956) 12 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7487 (408) 432-1900 q FAX: (408) 434-0507 q TELEX: 499-3977 U Dimensions in inches (milimeters) unless otherwise noted. T Package 5-Lead TO-220 0.169 – 0.185 (4.293 – 4.699) 0.079 – 0.135 (2.007 – 3.429) 0.035 – 0.055 (0.889 – 1.397) 0.460 – 0.500 (11.68 – 12.70) 0.620 ± 0.020 (15.75 ± 0.508) 0.700 – 0.728 (17.780 – 18.491) 0.970 – 1.050 (24.64 – 26.67) 0.015 – 0.025 (0.381 – 0.635) 0.210 – 0.240 (5.334 – 6.096) 0.055 – 0.090 (1.397 – 2.286) 0.079 – 0.115 (2.007 – 2.921) T5 (FORMED) 0392 0.028 – 0.035 (0.711 – 0.889) LT/GP 0193 10K REV 0 © LINEAR TECHNOLOGY CORPORATION 1993