HCV1707R1-R48-R datasheet(34/41 Pages) VICOR

34 / 41 page

Cool-Power® ZVS Switching Regulators

Rev 1.5

Page 34 of 41

03/2018

PI352x-00

Application Description

Output Voltage Set Point

The PI352x-00 family of Buck Regulators utilizes VREF, an internal

reference for regulating the output voltage. The output voltage

setting is accomplished using external resistors as shown in

Figure 93. Select R2 to be at or around 1kΩ for best noise

immunity. Use Equations 2 and 3 to determine the proper value

based on the desired output voltage.

Note: When using the above method of trimming by adjusting the value

of R1, the compensation of the control loops is modified and additional

Cout may be needed depending on the model. When the PI3526-00-LGIZ

is trimmed below 10V, the effective COUT must be at least 120µF, including

tolerance and voltage coefficient.

Soft Start Adjust and Tracking

The TRK pin offers a means to increase the regulator’s soft-start

time or to track with additional regulators. The soft-start slope

is controlled by an internal capacitor and a fixed charge current

to provide a Soft-Start Time tSS for all PI352x-00 regulators.

By adding an additional external capacitor to the TRK pin, the

soft-start time can be increased further. The following equation

can be used to calculate the proper capacitor for a desired

soft-start time in excess of tSS:

where tTRK is the soft-start time and ITRK is a 50µA internal charge

current (see Electrical Characteristics for limits).

In applications such as battery or super-capacitor charging

where the load is pre-biased, the PI352x can start into output

voltages up to the externally applied trim set point, or the

minimum absolute OVP, provided the value does not exceed

6V. For start up into loads which are pre-biased above 6V, an

ORing FET or equivalent sub-circuit is required to decouple the

buck output from the load during start up. In any application

with a CV type load, the regulator must be configured in a

constant-current mode of operation; the built-in current limit is a

fault protection only.

There is typically either proportional or direct tracking

implemented within a design. For proportional tracking between

several regulators at start up, simply connect all PI352x-00 device

TRK pins together. This type of tracking will force all connected

regulators to start up and reach regulation at the same time

(see Figure 94a).

For Direct Tracking, choose the PI352x-00 with the highest output

voltage as the master and connect the master to the TRK pin of

the other PI352x-00 regulators through a divider (Figure 95) with

the same ratio as the slave’s feedback divider.

All connected PI352x-00 regulator soft-start slopes will track

with this method. Direct tracking timing is demonstrated in

Figure 94b. All tracking regulators should have their Enable (EN)

pins connected together to work properly.

Figure 93 — External resistor divider network

EAO

EAIN

REF

EAIN-INT

COMP

OUT

OUT = VREF •

R1 + R2

(2)

R1 = R2 •

OUT – VREF

REF

(3)

where V

REF = VEAIN

(4)

TRK = (tTRK • ITRK ) – CTRK_INT

Figure 94 — PI352x-00 tracking responses

OUT 1

OUT 2

Master V

OUT

OUT 2

(a)

(b)

Figure 95 — Voltage divider connections for direct tracking

Master V

OUT

SGND

TRK

PI352x

Slave

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