Datasheet

PWM

VCC

COMP

VOUT

LM5001

SHUTDOWN

& STANDBY

SOFT-START

CAPACITOR

1.3V

1.26V

LM5001

LM5001-Q1

SNVS484G –JANUARY 2007–REVISED APRIL 2014

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Application Information (continued)

Figure 17. Soft-Start

8.2 Typical Applications

Figure 18, Figure 19, Figure 20, Figure 21, and Figure 22 present examples of a Non-Isolated Flyback, Isolated

Flyback, Boost, 24-V SEPIC and a 12-V Automotive range SEPIC converters utilizing the LM5001 switching

regulator.

8.2.1 Non-Isolated Flyback

The Non-Isolated Flyback converter (Figure 18) utilizes the internal voltage reference for the regulation setpoint.

The output is 5 V at 1 A while the input voltage can vary from 16 V to 42 V. The switching frequency is set to 250

kHz. An auxiliary winding on transformer (T1) provides 7.5 V to power the LM5001 when the output is in

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

voltage threshold is 13.9 V. The converter can be shut down by driving the EN input below 1.26 V 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 kΩ 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

LM5001 gradually charges the output capacitor by current limiting at approximately 1 A (I

LIM

) until regulation is

achieved.

Product Folder Links: LM5001 LM5001-Q1