PI3749-00-EVAL1 Datasheet, PDF (17/26 Pages) Vicor Corporation – Cool-Power® ZVS Switching Regulators

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.
( ) ^{R1 = R2 •}
V_OUT
1.7
-1
(1)
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, t_TRK is the desired soft-start time and I_SS is the T_RK pin
source current (see Electrical Characteristics for limits).
C_TRK =
(t_TRK • I_SS )
1.7
– 47 • 10 ^-9
(2)
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
V_OUT 1
V_OUT 2
(a)
Master V_OUT
V_OUT 2
(b)
t
Figure 25 — PI3749-x0 tracking methods
Cool-Power^® ZVS Switching Regulators
Page 17 of 26
Rev 1.8
03/2017
PI3749-x0
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).
Master V_OUT
PI3749
R1
TRK
Slave
R2
SGND
Figure 26 — Voltage divider connections for direct tracking
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 480nH 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 13) 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 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 3in. x 3in., 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
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