PI3420-00-LGIZ Datasheet, PDF (24/30 Pages) Vicor Corporation – 8 V to 18 Vin, 15 A Cool-Power ZVS Buck Regulator

R4
R1
-
+
R2
1.0 Vdc
V_OUT
REM
R_low
ADJ
SGND
R_high
Figure 38 — Internal resistor divider network
R1, R2, and R4 are all internal 1.0 % resistors and R_low and
R_high are external resistors for which the designer can add
to modify V_OUT to a desired output. The internal resistor
values for each regulator are listed next in Table 3.
Device
PI3420-00-LGIZ
PI3421-00-LGIZ
PI3422-00-LGIZ
PI3423-00-LGIZ
PI3424-00-LGIZ
R1
R2
R4
1k
∞
100
0.806 k
1.0 k
100
1.5 k
1.0 k
100
2.61 k
1.13 k
100
4.53 k
1.13 k
100
Table 3 — PI34xx-00 Internal divider values
By choosing an output voltage value within the ranges stated
in Table 2, V_OUT can simply be adjusted up or down by
selecting the proper R_high or R_low value, respectively. The
following equations can be used to calculate R_high and
R_low values:
If, for example, a 4.0 V output is needed, the user should
choose the regulator with a trim range covering 4.0 V from
Table 2. For this example, the PI3423 is selected (3.3 V set
voltage). First step would be to use Equation (1) to calculate
R_{_HIGH} since the required output voltage is higher than the
regulator set voltage. The resistor-divider network values for
the PI3423 can be found in Table 3 and are
R1 = 2.61k Ω and R2 = 1.13 kΩ. Inserting these values in to
Equation (1), R_{_HIGH} is calculated as follows:
Cool-Power®
Page 24 of 30
Rev 1.3
12/2015
PI34xx-00
Resistor R_high should be connected as shown in Figure 38
to achieve the desired 4.0 V regulator output. No external
R_low resistor is need in this design example since the trim is
above the regulator set voltage.
The PI3420 output voltage can only be trimmed higher than
the factory 1 V setting. The following Equation (3) can be
used calculate Rhigh values for the PI3420 regulators.


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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 for all
PI34xx-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 times:
C_TRK = (t_TRK x I_TRK) – 100 x 10 ^-9,
where, t_TRK is the soft-start time and I_TRK is a 50 μA internal
charge current (see Electrical Characteristics for limits).
There is typically either a proportional or direct tracking
method implemented within a tracking design. For
proportional tracking between several regulators at startup,
simply connect all devices TRK pins together. This type of
tracking will force all connected regulators to startup and
reach regulation at the same time (see Figure 39 (a)).
For Direct Tracking, choose the regulator with the highest
output voltage as the master and connect the master TRK pin
to the TRK pin of the other regulators through a divider
(Figure 40) with the same ratio as the slave’s feedback
divider (see Table 3 for values).
V_OUT 1
V_OUT 2
(a)
Master V_OUT
V_OUT 2
(b)
t
Figure 39 — PI34xx-00 tracking methods
All connected regulators’ soft-start slopes will track with this
method. Direct tracking timing is demonstrated in
Figure 39 (b).
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