LT1737 Linear Technology Corporation, LT1737 Datasheet
LT1737
Available stocks
Related parts for LT1737
LT1737 Summary of contents
Page 1
... Its gate drive capability, coupled with a suitable external MOSFET and other power path components, can deliver load power up to tens of Watts. The LT1737 has a number of features not found on other isolated flyback controller ICs. By utilizing current mode switching techniques, it provides excellent AC and DC line regulation ...
Page 2
... UVLO Pin Voltage .................................................... V I Pin Voltage .................................................... 2V SENSE FB Pin Current ..................................................... 2mA Operating Junction Temperature Range LT1737C ............................................... 100 C LT1737I ............................................ – 125 C Storage Temperature Range ................ – 150 C Lead Temperature (Soldering, 10 sec)................ 300 C ELECTRICAL CHARACTERISTICS temperature range, otherwise specifications are at T SYMBOL PARAMETER Power Supply ...
Page 3
... Duty Cycle = Min Duty Cycle 30% Duty Cycle 30% Duty Cycle = 80 100pF OSCAP (Note 50k tON R = 50k ENDLY R = 50k MENAB (Note 1.2V UVLO V = 1.3V UVLO I = 1mA LOAD LT1737 MIN TYP MAX UNITS 0.90 1.12 1.25 V 0.80 1.35 V 220 250 270 mV 200 280 mV 220 mV 0. 100 115 kHz ...
Page 4
... LT1737 W U TYPICAL PERFOR A CE CHARACTERISTICS Minimum Input Voltage vs Temperature 4.3 4.2 4.1 4.0 3.9 3.8 –50 – 100 125 TEMPERATURE ( C) 1737 G01 Supply Current vs Temperature – 125 –25 100 TEMPERATURE ( C) 1737 G04 GATE SINK 1.0 0 125 – ...
Page 5
... T = 125 C A 600 400 200 1.25 1.30 1.35 1.40 1737 G13 2.5 V(SFST 2.0 1.5 1.0 0 100 125 1737 G15 LT1737 Enable Delay Time vs Temperature 275 250 225 200 175 150 125 125 25 50 –50 –25 0 TEMPERATURE ( C) 1737 G11 Feedback Amplifier Transconductance vs Temperature –50 – ...
Page 6
... Pin for external programming resistor to set ON switch minimum on time. See Applications Information for details. V (Pin 15): Supply voltage for the LT1737. Bypass this CC pin to ground with more. GATE (Pin 16): This is the gate drive to the external power MOSFET switch and has large dynamic currents flowing through it ...
Page 7
... W BLOCK DIAGRA V CC UVLO BIAS OSCAP OSC FB FDBK 3V OUT 3V REG (INTERNAL) t MINENAB ENDLY ON LOGIC COMP SOFT-START V SFST C LT1737 GATE MOSFET DRIVER PGND I SENSE I AMP SGND LOAD COMPENSATION 1737 OCMP CMPC 1737f 7 ...
Page 8
... LT1737 DIAGRA V SW VOLTAGE V IN GND SWITCH OFF STATE MINIMUM t ON FLYBACK AMP STATE W FLYBACK ERROR A PLIFIER OFF ENABLE DELAY DISABLED MINIMUM ENABLE TIME T1 • D1 • • FXD COLLAPSE ...
Page 9
... U OPERATIO The LT1737 is a current mode switcher controller IC designed specifically for the isolated flyback topology. The Block Diagram shows an overall view of the system. Many of the blocks are similar to those found in traditional designs, including: Internal Bias Regulator, Oscillator, Logic, Current Amplifier and Comparator, Driver and Out- put Switch ...
Page 10
... These include effective transconductance and V LOAD COMPENSATION THEORY The LT1737 uses the flyback pulse to obtain information about the isolated output voltage. A potential error source is caused by transformer secondary current flow through the real life nonzero impedances of the output rectifier, ...
Page 11
... SENSE R = uncompensated output impedance OUT (R1||R2) = impedance of R1 and R2 in parallel The practical aspects of applying this equation to deter- mine an appropriate value for the R in the Applications Information section. LT1737 . This voltage is then CMPC resistor by op amp OCMP R SENSE • ...
Page 12
... APPLICATIO S I FOR ATIO TRANSFORMER DESIGN CONSIDERATIONS Transformer specification and design is perhaps the most critical part of applying the LT1737 successfully. In addi- tion to the usual list of caveats dealing with high frequency isolated power supply transformer design, the following information should prove useful. ...
Page 13
... Verify this result by connecting a resistor of roughly this value from the R ground short to R filter capacitor to ground.) Measure the output impedance with the new compensation in place. Modify the original R value if necessary to increase or decrease the OCMP effective compensation. LT1737 RESISTOR VALUE OCMP , i.e., effective output impedance and OUT 1 ESR 1 – ...
Page 14
... U U APPLICATIO S I FOR ATIO SELECTING OSCILLATOR CAPACITOR VALUE The switching frequency of the LT1737 is set by an external capacitor connected between the OSCAP pin and ground. Recommended values are between 200pF and 33pF, yielding switching frequencies between 50kHz and 250kHz. Figure 2 shows the nominal relationship between external capacitance and switching frequency ...
Page 15
... Note that the instantaneous peak power in the sense resistor is 2.5W, and it must be FLYBACK WAVEFORM WITH LARGE LEAKAGE rated accordingly. The LT1737 has only a single sense line SPIKE AT HEAVY LOAD to this resistor. Therefore, any parasitic resistance in the ground side connection of the sense resistor will increase its apparent value ...
Page 16
... L P PARASITIC INDUCTANCE 1737 F05 Figure 5 SOFT-START FUNCTION The LT1737 contains an optional soft-start function that is enabled by connecting an explicit external capacitor be- tween the SFST pin and ground. Internal circuitry prevents the control voltage at the V pin from exceeding that on the C SFST pin. The soft-start function is enagaged whenever V is removed result of either undervoltage lockout or thermal (overtemperature) shutdown ...
Page 17
... In further contrast to traditional current mode switchers, V pin ripple is generally not an issue with the LT1737. The C dynamic nature of the clamped feedback amplifier forms an effective track/hold type response, whereby the V voltage changes during the flyback pulse, but is then “held” ...
Page 18
... These relate to the mini- mum switch on time and the minimum enable time. Discontinuous mode operation will be assumed in the following theoretical derivations. As outlined in the Operation section, the LT1737 utilizes a minimum output switch on time, t combined with expected V and switching frequency to IN yield an expression for minimum delivered power ...
Page 19
... U U APPLICATIO S I FOR ATIO MAXIMUM LOAD/SHORT-CIRCUIT CONSIDERATIONS The LT1737 is a current mode controller. It uses the V node voltage as an input to a current comparator that turns off the output switch on a cycle-by-cycle basis as this peak current is reached. The internal clamp on the V nominally 2 ...
Page 20
... W U GATE DRIVE RESISTOR CONSIDERATIONS The gate drive circuitry internal to the LT1737 has been designed to have as low an output impedance as practi- cally possible—only a few ohms. A strong L/C resonance is potentially presented by the inductance of the path leading to the gate of the power MOSFET and its overall gate capacitance ...
Page 21
... D1: MOTOROLA 30V, 3A SCHOTTKY RECTIFIER D2: 20V, 500mW ZENER DIODE D3: MOTOROLA 40V, 0.5A SCHOTTKY RECTIFIER the LT1737 and the gate of MOSFET M1, an LT1121 low dropout linear regulator is employed (U2). Resistor di- vider R11/R12 sets the output nominally 8.25V. (A few hundred millivolts of dropout will therefore be seen at ...
Page 22
... U2 as input voltage is increased. The LT1725, a close “cousin” of the LT1737 is recommended in such instances. The LT1725 is very similar to the LT1737, but it contains an integral wide hysteresis undervoltage lockout (UVLO) circuit that monitors the V conjunction with a 3-winding transformer to provide both device power and output voltage feedback information, this allows for a “ ...
Page 23
... Figure 13. Efficiency vs I make this a preferable alternative. Furthermore, a variety of manufacturers offer off-the-shelf dual wound magnet- ics which often can be applied as 1:1 transformers.) Figure 12 shows an LT1737 configured for operation with a dual wound toroid, the Coiltronics #CTX150-4 OCTA-PAC voltage waveform is now provided by components Q1, R2, OCTA-PAC is a trademark of Coiltronics, Inc ...
Page 24
... T1: COILTRONICS TRANSFORMER 24 5V APPLICATION IN The LT1737 is a bipolar technology IC specified to operate down to a minimum input supply voltage of 4.5V. Although its GATE pin drives “low” nearly to ground, its “high” capability is limited by a headroom requirement of roughly 2V s. Thus when operating at a worst case 4.5V supply, BE the GATE output will only drive nominal ...
Page 25
... R dropout regulator is programmed to a nominal 8.25V 10 output by the R12/R13 resistor divider, and this allows the LT1737 to drive M1, a logic level MOSFET. Minimum on 1737 F16 time programming resistor R5 is set to 33k to minimize the LOAD required output preload. Minimum enable time has no direct effect on steady state operation, but programming resistor R7 has been set to 100k for rapid start-up ...
Page 26
... LT1737 PACKAGE DESCRIPTIO 0.007 – 0.0098 (0.178 – 0.249) 0.016 – 0.050 (0.406 – 1.270) * DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE ** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE ...
Page 27
... DWG # 05-08-1610 0.228 – 0.244 (5.791 – 6.197 0.053 – 0.069 (1.346 – 1.752) 0 – 8 TYP 0.014 – 0.019 (0.355 – 0.483) TYP LT1737 0.386 – 0.394* (9.804 – 10.008 0.150 – 0.157** (3.810 – 3.988 ...
Page 28
... C1 150 F 1 OUT 20V R12 U2 24k LT1121 2 5% ADJ C4 R13 1 F 20k 25V 5% Z5U GATE LT1737 2 I SENSE C10 MINENAB R R SGND PGND OCMP CMPC 470pF 50V X7R R7 100k 5% Figure 18. 8V-16V to 12V Nonisolated Converter ...