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LTC7821

Rev A

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APPLICATIONS INFORMATION

To obtain the most translation in voltage from VINTVCC to

drive M1, Schottky diodes are recommended.

The reverse voltage seen by each of the Schottky diodes

is approximately:

VR _DIODE ≅

VIN

Pay attention to the leakage current when selecting the

forward drop of the diodes. In general, the lower the

forward drop for the same amount of current flowing

through the diode, the higher the leakage current. As

these diodes will be operating with large reverse bias for

high VIN applications, the leakages are higher, especially

at higher operating temperatures.

The charge pump diodes operate as conduits for transfer-

ring charge from one capacitor to another and as such are

subjected to transient current rather than a DC current.

Hence peak forward surge rating is more important than

the average current rating. A surge rating of 750mA is a

good starting point.

Since the bootstrap capacitor acts as a supply for the

MOSFET’s driver, select large enough capacitance so

that the voltage does not droop considerably when the

MOSFETs are being turned ON. As with the CMILLER esti-

mation described in the Power Selection section, the user

can use the same graph to obtain the total gate charge for

a given gate drive voltage. This can then easily converted

to its equivalent gate capacitance by:

CG =

VGS

If the bootstrap capacitor voltage is not allowed to droop

by more than 1%, then:

CBST ≥ 99CG

Besides acting as a supply for their respective MOSFET

drivers, CBST2 and CBST3 also serve as charge pump ca-

pacitors. As a good starting point, size CBST2 and CBST3

as follows:

CBST3 ≥ 2CBST2 ≥ 2CBST1

Soft-Start and Tracking

The LTC7821 has the ability to either soft-start by itself

with a capacitor or track the output of another channel or

external supply. When configured to soft-start by itself, a

capacitor should be connected to its TRACK/SS pin and

ground.WhenRUNpinvoltageisbelow1.22V,theTRACK/

SS is actively pulled to ground in this shutdown state.

Once the RUN pin voltage is above 1.22V, the controller

powers up and a soft-start current of 10µA begins to flow

outoftheTRACK/SSpin.However,theTRACK/SSwillstart

charging its soft-start capacitor only after charge balance

is completed and the associated active pull-down of the

TRACK/SS is released. Note that soft-start or tracking is

achieved not by limiting the maximum output current of

the controller but by controlling the output ramp voltage

according to the ramp rate on the TRACK/SS pin. Current

fold-back is disabled during this phase to ensure smooth

soft-start or tracking. Depending on whether the EXT_REF

feature has been invoked or not, the soft-start or tracking

range is defined to be either the voltage range from 0V to

0.8V or 0V to VEXT_REF on the TRACK/SS pin. The total

soft-start time can be calculated as:

tSOFT −START = 0.8 or VEXT_REF

(

) •

CSS

10µA

Regardless of the mode selected by the MODE/PLLIN pin,

the regulator will always start in pulse-skipping mode up

to TRACK/SS = 82.5% of 0.8V or VEXT_REF.

Output Voltage Tracking

The LTC7821 allows the user to program how its output

ramps up and down by means of the TRACK/SS pins.

Through this pin, the output can be set up to either coin-

cidentallyorratio-metricallytrackanothersupply’soutput,

as shown in Figure 15. In the following discussions, VOUT1

refers to another supply’s output (master channel) while

VOUT2 refers to the LTC7821 output (slave channel) that

tracksVOUT1.ToimplementthecoincidenttrackinginFigure

15a, connect an additional resistive divider to VOUT1 and

connect its midpoint to the TRACK/SS pin of the LTC7821.

The ratio of this divider should be the same as that of the

slave channel’s feedback divider shown in Figure 16a. In

this tracking mode, VOUT1 must be set higher than VOUT2.

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