lt3750 Linear Technology Corporation, lt3750 Datasheet
lt3750
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lt3750 Summary of contents
Page 1
... LT3750 can operate from a variety of power sources. A typical application can charge a 100µF capacitor to 300V in less than 300ms. The CHARGE pin gives full control of the LT3750 to the user. The DONE pin indicates when the capacitor has reached its programmed value and the part has stopped charging ...
Page 2
... GATE 1nF GATE Note 2: The LT3750E is guaranteed to meet performance specifications from 0°C to 70°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. Note 3: Refer to Block Diagram for V U ...
Page 3
... TEMPERATURE (°C) 3750 G05 V Comparator Trip Voltage OUT 1.240 1.236 1.232 1.228 1.224 1.220 –50 – 100 125 TEMPERATURE (°C) 3750 G08 LT3750 CHARGE Pin Current 45 40 –50° 25° 125° ...
Page 4
... LT3750 CTIO S V (Pin 1): Transformer Supply Pin. Powers the TRANS primary coil of the transformer as well as internal circuitry that performs boundary mode detection. Bypass at the pin with a 1µF to 10µF capacitor. Bypass the primary winding of the transformer with a large capacitor. ...
Page 5
... M1 ENABLE CHARGE ONE 3 SHOT 1 V TRANS COMPARATOR ONE SHOT + – 1.24V V OUT COMPARATOR CURRENT LIMIT COMPARATOR GND RBG LT3750 V TRANS T1 1:N • VOUT OUT 9 DCM R DCM RDCM + 8 + – – V TRANS 36mV V DRAIN GATE R Q ...
Page 6
... LT3750 U OPERATIO The LT3750 is designed to charge capacitors quickly and efficiently. Operation can be best understood by referring to Figures 1 and 2. Operation proceeds in four phases: 1. Start-up, 2. Primary-side charging, 3. Secondary en- ergy transfer, 4. Discontinuous mode sensing. 1. Start-Up Start-up occurs for approximately 20µs after the charge pin is raised high. During this phase, a one-shot enables the master latch and turns on the NMOS ...
Page 7
... NMOS will ring down from TRANS OUT voltage falls to V the NMOS on-latch and a new charge cycle begins. Steps 2-4 continue until the target output voltage is reached. LT3750 comparator resets the master OUT )/N will appear across the primary winding )/ When the drain DIODE ...
Page 8
... NMOS, the secondary side of the transformer, and the output. Transformer Selection The flyback transformer is critical to proper operation of the LT3750. It must be designed carefully so that it does not cause excessive current or voltage on any pin of the part. As with all circuits, the LT3750 has finite bandwidth. In ...
Page 9
... This substantially reduces the efficiency and increases the ) charge time. The second way is when the LT3750 fails to detect discontinuous mode after the first switching cycle. Both of these problems are solved by increasing the amount of capacitive bypassing for the transformer. Choose ca- pacitors that can handle the high RMS ripple currents common in flyback regulators ...
Page 10
... DELAY PRI applications. Higher output voltages will require a larger resistor. In order for the LT3750 to properly detect discontinuous mode and start a new charge cycle, the reflected voltage to the primary winding must exceed the discontinuous mode comparator threshold which is nominally 36mV. The worst-case condition occurs when V ground ...
Page 11
... U U APPLICATIO S I FOR ATIO Board Layout The high voltage operation of the the LT3750 demands careful attention to board layout. Observe the following points: 1. Minimize the area of the high voltage end of the second- ary winding. 2. Provide sufficient spacing for all high voltage nodes ...
Page 12
... V 60.4k TRANS OUT C1 100k 43k 10µF DONE RDCM LT3750 OFF ON CHARGE GATE SOURCE GND RBG 2.49k 33pF 3750 TA02a C1: 25V X5R OR X7R CERAMIC CAPACITOR C2: 25V X5R OR X7R CERAMIC CAPACITOR C3: 25V SANYO OS-CON 25SVP56M C4: 330V RUBYCON PHOTOFLASH CAPACITOR D1: DIODES INC. MURS160 ...
Page 13
... Charge Time 300 V = 18V TRANS 250 TRANS 200 V = 12V TRANS 150 100 0.1 0.2 0.3 0.4 TIME (SECONDS) 3750 TA03c LT3750 T1 D1 1:10 V OUT 300V 1 • 100µF • 12mΩ Typical Switching Waveforms NMOS DRAIN CURRENT 2A/DIV NMOS DRAIN VOLTAGE 20V/DIV 5µs/DIV ...
Page 14
... V 60.4k TRANS OUT C1 100k 43k 10µF DONE RDCM LT3750 CHARGE GATE OFF ON SOURCE GND RBG 2.49k 100pF C1: 25V X5R OR X7R CERAMIC CAPACITOR C2: 25V X5R OR X7R CERAMIC CAPACITOR C3: 25V SANYO OS-CON 25SVP56M C4: 330V RUBYCON PHOTOFLASH CAPACITOR D1: DIODES INC. MURS160 M1: PHILIPS PHM2INQ15T ...
Page 15
... TYP 0.53 ± 0.152 (.021 ± .006) DETAIL “A” SEATING PLANE 0.17 – 0.27 (.007 – .011) TYP LT3750 0.497 ± 0.076 (.0196 ± .003 REF 3.00 ± 0.102 (.118 ± .004) (NOTE ...
Page 16
... V 60.4k TRANS OUT C1 100k 43k 10µF DONE RDCM LT3750 OFF ON CHARGE GATE SOURCE GND RBG 2.49k 100pF 3750 TA05a D1: DIODES INC. MURS160 M1: PHILIPS PHM21NQ15T T1: MIDCOM 32052 FLYBACK TRANSFORMER 250 300 3750 TA05b COMMENTS Charges 220µF to 320V in 3.7 Seconds from 5V < ...