LTC3370 datasheet(28/44 Pages) LINER

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LTC3372

Rev. A

For more information www.analog.com

LTC3372

TRACK/SS

CSS

SGND

3372 F05

Figure 5. Using the TRACK/SS Pin to Program Soft-Start

APPLICATIONS INFORMATION

Tracking and Soft-Start (TRACK/SS Pin)

The start-up of VOUT is controlled by the voltage on the

TRACK/SS pin. When the voltage on the TRACK/SS pin

is less than the internal 1.2V reference, it regulates the

internal VFB voltage to the voltage on the TRACK/SS pin

insteadof1.2V.TheTRACK/SSpincanbeusedtoprogram

an external soft-start function or to allow VOUT to track

another supply during start-up.

Soft-start is enabled by simply connecting a capacitor

from the TRACK/SS pin to ground, as shown in Figure 5.

An internal 10μA current source charges the capacitor,

providing a linear ramping voltage at the TRACK/SS pin.

The HV controller will regulate the VFB (and hence VOUT)

according to the voltage on the TRACK/SS pin, allowing

VOUT/EXTVCCtorisesmoothlyfrom0Vtoitsfinalregulated

value. The total soft-start time will be approximately:

tSS = CSS •

1.2V

10µA

Alternatively,theTRACK/SSpincanbeusedtotrackanother

supplyduringstart-up,asshownqualitativelyinFigures6a

and 6b. To do this, a resistor divider should be connected

from the master supply (VX) to the TRACK/SS pin of the

slave supply (VOUT/EXTVCC) as shown in Figure 7. During

start-up VOUT/EXTVCC will track VX according to the ratio

set by the resistor divider:

VOUT

1.2V

VOUT(REG)

•

RTRACKA +RTRACKB

RTRACKA

VOUT(REG) is the programmed output regulation voltage.

If the VOUTPRG pin it tied to INTVCC pin, VOUT(REG) = 5V.

If the VOUTPRG pin is tied to ground, VOUT(REG) = 3.3V.

INTVCC Regulation

INTVCC powers the gate drivers and much of the internal

circuitry. Power to the INTVCC pin is supplied either from

the VINpinthroughaninternallowdropoutlinearregulator

(LDO), or from the VOUT/EXTVCC pin through an internal

switch, depending on the voltage at VOUT/EXTVCC. The VIN

LDO regulates INTVCC to 5.1V. The INTVCC can supply a

peak current of at least 50mA and must be bypassed to

ground with a minimum of 4.7μF ceramic capacitor. Good

LTC3372

VOUT/EXTVCC

TRACK/SS

3372 F07

RTRACKA

RTRACKB

Figure 7. Using the TRACK/SS Pin for Tracking

TIME

VX(MASTER)

VOUT(SLAVE)

3372 F06a

TIME

3372 F06b

VX(MASTER)

VOUT(SLAVE)

Figure 6. Two Different Modes of Output Voltage Tracking

(6a) Coincident Tracking

(6b) Ratiometric Tracking

High Voltage Buck Controller

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