LM5002

SNVS496D – JANUARY 2007 – REVISED MARCH 2013

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Application Circuit Examples

The following schematics present examples of a Non-Isolated Flyback, Isolated Flyback, Boost, 24V SEPIC and

a 12V Automotive range SEPIC converters utilizing the LM5002 switching regulator.

NON-ISOLATED FLYBACK

The Non-Isolated Flyback converter (Figure 21) utilizes the internal voltage reference for the regulation setpoint.

The output is +5V at 500mA while the input voltage can vary from 16V to 42V. The switching frequency is set to

250kHz. An auxiliary winding on transformer (T1) provides 7.5V to power the LM5002 when the output is in

regulation. This disables the internal high voltage VCC LDO regulator and improves efficiency. The input under-

voltage threshold is 13.9V. The converter can be shut down by driving the EN input below 1.26V with an open-

collector or open-drain transistor. An external synchronizing frequency can be applied to the SYNC input. An

optional soft-start circuit is connected to the COMP pin input. When power is applied, the soft-start capacitor (C7)

is discharged and limits the voltage applied to the PWM comparator by the internal error amplifier. The internal

~5 k

Ω COMP pull-up resistor charges the soft-start capacitor until regulation is achieved. The VCC pull-up

resistor (R7) continues to charge C7 so that the soft-start circuit will not affect the compensation network in

normal operation. If the output capacitance is small, the soft-start circuit can be adjusted to limit the power-on

output voltage overshoot. If the output capacitance is sufficiently large, no soft-start circuit is needed because the

regulation is achieved.

ISOLATED FLYBACK

The Isolated Flyback converter (Figure 22) utilizes a 2.5V voltage reference (LM431) located on the isolated

secondary side for the regulation setpoint. The LM5002 internal error amplifier is disabled by grounding the FB

pin. The LM431 controls the current through the opto-coupler LED, which sets the COMP pin voltage. The R4

and C3 network boosts the phase response of the opto-coupler to increase the loop bandwidth. The output is

+5V at 500mA and the input voltage ranges from 16V to 42V. The switching frequency is set to 250kHz.

BOOST

The Boost converter (Figure 23) utilizes the internal voltage reference for the regulation setpoint. The output is

+48V at 125 mA, while the input voltage can vary from 16V to 36V. The switching frequency is set to 250kHz.

The internal VCC regulator provides 6.9V bias power, since there isn’t a simple method for creating an auxiliary

voltage with the boost topology. Note that the boost topology does not provide output short-circuit protection

because the power MOSFET cannot interrupt the path between the input and the output.

24V SEPIC

The 24V SEPIC converter (Figure 24) utilizes the internal voltage reference for the regulation setpoint. The

output is +24V at 125 mA while the input voltage can vary from 16V to 48V. The switching frequency is set to

250kHz. The internal VCC regulator provides 6.9V bias power for the LM5002. An auxiliary voltage can be

created by adding a winding on L2 and a diode into the VCC pin.

12V AUTOMOTIVE SEPIC

The 12V Automotive SEPIC converter (Figure 25) utilizes the internal bandgap voltage reference for the

regulation setpoint. The output is +12V at 25 mA while the input voltage can vary from 3.1V to 60V. The output

current rating can be increased if the minimum VIN voltage requirement is increased. The switching frequency is

set to 750kHz. The internal VCC regulator provides 6.9V bias power for the LM5002. The output voltage can be

used as an auxiliary voltage if the nominal VIN voltage is greater than 12V by adding a diode from the output into

the VCC pin. In this configuration, the minimum input voltage must be greater than 12V to prevent the internal

VCC to VIN diode from conducting. If the applied VCC voltage exceeds the minimum VIN voltage, then an

external blocking diode is required between the VIN pin and the power source to block current flow from VCC to

the input supply.

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