PT7720_13 Datasheet, PDF (5/8 Pages) Texas Instruments – High Efficiency

Application Notes
PT7720 Series
Pin-Coded Output Voltage Adjustment on the
“Big Hammer II” Series ISRs
Power Trends PT7720 series ISRs incorporate pin-coded
voltage control to adjust the ouput voltage. The control
pins are identified VID0 - VID4 (pins 1, 2, 3, 4, & 6)
respectively. When the control pins are left open-circuit,
the ISR will regulate at its factory trimmed output voltage.
Each pin is internally connected to a precision resistor,
which when grounded changes the output voltage by a
set amount. By selectively grounding VID0-VID4, the
output voltage of each ISR in the PT7720 series ISRs
can be programmed in incremental steps over its speci-
fied output voltage range. The output voltage ranges
offered by these regulators provide a convenient method
of voltage selection for many applications. In addition,
the program code and output voltage range of the PT7721
model is compatible with the voltage ID specification
defined by Intel Corporation for voltage regulator modules
(VRMs) used to power Pentium® II microprocessors.
Refer to Figure 2-1 below for the connection schematic,
and the PT7720 Data Sheet for the appropriate pro-
gramming code information.
Notes:
1. The programming convention is as follows:-
Logic 0: Connect to pin12 (Remote Sense Ground).
Logic 1: Open circuit/open drain (See notes 2, & 4)
2. Do not connect pull-up resistors to the voltage
programming pins.
3. To minimize output voltage error, always use pin 12 (Sense
Ground) as the logic “0” reference. While the regular
ground (pins 13-19) can also be used for programming,
doing so will degrade the load reglation of the product.
4. If active devices are used to ground the voltage control
pins, low-level open drain MOSFET devices should be
used over bipolar transistors. The inherent V_ce(sat) in
bipolar devices introduces errors in the device’s internal
divider network. Discrete transistors such as the BSS138,
2N7002, or the IRLML2402 are examples of appropriate
devices.
Active Voltage Programming:
Special precautions should be taken when making changes
to the voltage control progam code while the unit is
powered. It is highly recommended that the ISR be either
powered down or held in standby. Changes made to the
program code while V_out is enabled induces high current
transients through the device. This is the result of the
electrolytic output capacitors being either charged or dis-
charged to the new output voltage set-point. The transient
current can be minimized by making only incremental
changes to the binary code, i.e. one LSB at a time. A
minimum of 100µs settling time between each program
state is also recommended. Making non-incremental
changes to VID3 and VID4 with the output enabled is
discouraged. If they are changed, the transients induced
can overstress the device resulting in a permanent drop
in efficiency. If the use of active devices prevents the
program code being asserted prior to power-up, pull pin
5 (STBY) to the device GND during the period that the
input voltage is applied to V_in. Releasing pin 5 will then
allow the device output to execute a soft-start power-up
to the programmed voltage.
Figure 2-1
+12V
STBY
COM
1µH
(Optional)
64321
VID4 - VID0
7-11
Vin
PT7720
Synch
STBY _Out
5 27
GND
13-19
26
SNS(+)
20-25
Vo
SNS(-)
12
+
Cin
L
+
Cout
O
A
D
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