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LTC4091

4091fa

For more information www.linear.com/LTC4091

High Voltage Regulator Burst Mode Operation

At higher output loads (above 90mA for the back page

application) the LTC4091 will be running at the frequency

programmed by the RT resistor, and will be operating in

standard PWM mode. The transition between PWM and

low ripple Burst Mode operation is seamless, and will not

disturb the output voltage.

Boost Pin Considerations

Capacitor C2 (see Block Diagram) and an internal diode

are used to generate a boost voltage that is higher than

the input voltage. In most case, a 0.47µF capacitor will

work well. The BOOST pin must be at least 2.3V above

the SW pin for proper operation.

High Voltage Regulator Soft-Start

The RUN/SS pin can be used to soft-start the high voltage

regulatoroftheLTC4091,reducingmaximuminputcurrent

during start-up. The RUN/SS pin is driven through an ex-

ternal RC filter to create a voltage ramp at this pin. Figure

6 shows the start-up and shut-down waveforms with the

soft-start circuit. By choosing a large RC time constant

the peak start-up current can be reduced to the current

that is required to regulate the output with no overshoot.

Choose the value of the resistor so that it can supply 20µA

when the RUN/SS pin reaches 2.3V.

Automotive Cold-Crank Ride Through

In automotive applications that are expected to experience

large dips in supply voltage, such as during a cold-crank

event, the high voltage switching regulator can lose regu-

lation resulting in excessive VC voltage and consequently

excessive output overshoot when VINrecovers.Toprevent

overshoot when recovering from a cold-crank event it is

necessary to reset the LTC4091's soft-start circuit via

the RUN/SS pin. Figure 7 shows an example of a simple

circuitthatautomaticallydetectsabrown-outconditionand

resets the RUN/SS pin, re-engaging the soft-start feature

and preventing damaging output overshoot.

APPLICATIONS INFORMATION

Synchronization and Mode

The SYNC pin allows the high voltage regulator to be

synchronized to an external clock.

Synchronizing the LTC4091 internal oscillator to an

external frequency can be done by connecting a square

wave (with 20% to 80% duty cycle) to the SYNC pin. The

square wave amplitude should be such that the valleys

are below 0.3V and the peaks are above 1V (up to 6V).

The high voltage regulator may be synchronized over a

300kHz to 2MHz range. The RT resistor should be chosen

such that the LTC4091 oscillates 25% slower than the

external synchronization frequency to ensure adequate

slopecompensation.Whilesynchronized,thehighvoltage

regulator will turn on the power switch on positive going

edges of the clock.

Figure 6. To Soft-Start the High Voltage Regulator,

Add a Resistor and Capacitor to the RUN/SS Pin

4091 F07

1A/DIV

VRUN/SS

2V/DIV

VHVOUT

2V/DIV

RUN/SS

GND

0.22µF

RUN

15k

2ms/DIV

Figure 7. Cold-Crank Ride Through Circuit

4091 F08

CMHZ4692

49.9k

100k

140k

1/2 MMDT5551

330nF

VIN

RUN/SS

GND

AUTOMOTIVE

INPUT

1/2 MMDT5551

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