PI3749 datasheet(17/26 Pages) VICOR

17 / 26 page

Cool-Power® ZVS Switching Regulators

Rev 1.6

vicorpower.com

Page 17 of 26

09/2016

800 927.9474

PI3749-x0

Applications Information

Input / Output Range Limitation

The PI3749-x0 is capable of wide step-up and step-down

conversions, but high boosting ratios place thermal stress on

the external inductor that may not be fully protected by the

controller overtemperature shut down. For this reason boosting

above 29V out when the input voltage is less than 24V

is not supported.

Output Voltage Trim

The output voltage can be adjusted by feeding back a portion

of the desired output through a voltage divider to the error

amplifier’s input (see Figure 24). Equation 1 can be used to

determine resistor values needed for the voltage divider.

The R2 value is selected by the user; a 1.07kΩ resistor value is

recommended.

If, for example, a 24V output is needed, the user can select a

1.07kΩ (1%) resistor for R2 and use equation (1) to calculate R1.

Once R1 value is calculated, the user should select the nearest

resistor value available. In this example, R1 is 14.03kΩ so a

14.0kΩ should be selected.

Soft-Start Adjustment 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 47nF and a fixed charge current to

provide a minimum startup time of 1.6ms (typical). By adding

an 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 times:

Where, tTRK is the desired soft-start time and ISS is the TRK pin

source current (see Electrical Characteristics for limits).

The PI3749-x0 allows the tracking of multiple like regulators.

Two methods of tracking can be chosen: proportional or direct

tracking. Proportional tracking will force all connected regulators

to startup and reach regulation at the same time (see Figure

25 (a)). To implement proportional tracking, simply connect all

devices TRK pins together.

For Direct Tracking, choose the regulator with the highest output

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

the other regulators through a divider (Figure 26) with the same

ratio as the slave’s feedback divider (see Output Voltage Trim).

All connected regulators’ soft-start slopes will track with this

method. Direct tracking timing is demonstrated in

Figure 25 (b). All tracking regulators should have their Enable

(EN) pins connected together for proper operation.

Inductor Pairing

Operations and characterization of the PI3749-x0 was performed

using a 480 nH inductor, Part # HCV1206-R48-R, manufactured

by Eaton. This Inductor has a form factor of 12.5mm x 10mm

x 5mm. No other inductor is recommended for use with the

PI3749-x0. For additional inductor information and sourcing,

please contact Eaton directly.

Thermal De-rating

Thermal de-rating curves are provided (page 14) that are based

on component temperature changes versus load current, input

voltage and no air flow. It is recommended to use these curves as

a guideline for proper thermal de-rating. These curves represent

the entire system and are inclusive to both the Picor SiP and

the external inductor. Maximum thermal operation is limited by

either the MOSFETs or inductor depending upon line and

load conditions.

All thermal testing was performed using a 3 in. x 3 in., four

2oz. copper layers, FR4 evaluation board platform. Thermal

measurements were made on the five main power devices; the

four internal MOSFETS and the external inductor.

Filter Considerations

The PI3749-x0 requires low impedance ceramic input capacitors

(X7R/X5R or equivalent) to ensure proper start up and high

frequency decoupling for the power stage. The PI3749-x0

will draw nearly all of the high frequency current from the

low impedance ceramic capacitors when the main high side

MOSFET(s) are conducting. During the time the MOSFET(s) are

off, the input capacitors are replenished from the source.

Table 1 shows the recommended input and output capacitors to

be used for the PI3749-x0 as well as total RMS current, and input

and output ripple voltages. Divide the total RMS current by the

R1 = R2 •

-1

(1)

OUT

1.7

()

TRK =

– 47 • 10-9

(2)

TRK • ISS )

1.7

VOUT 1

VOUT 2

Master VOUT

VOUT 2

(a)

(b)

Figure 25 — PI3749-x0 tracking methods

Master VOUT

SGND

TRK

PI3749

Slave

Figure 26 — Voltage divider connections for direct tracking

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