PT7681 Datasheet, PDF (8/12 Pages) Texas Instruments – 30-A Programmable ISR With Margin Up/Down Controls

Application Notes
PT7680 Series
Pin-Coded Output Voltage Adjustment of the
PT7680 Series of 30-A Switching Regulators
The PT7680 Excalibur™ family of converters use a pin
code to adjust the output voltage. This feature uses the
control inputs, VID0–VID3 (pins 1–4). When these pins
are left open-circuit, the ISR regulates at its default output
voltage. Each of the programming pins are internally
connected to a precision resistor, and when pulled low ¹
applies a weighted change to the output voltage. By
selectively connecting VID0–VID3 to Sense(–) ², the
output voltage of these ISRs can be programmed in
incremental steps over their specified output voltage
range. The program codes and output voltages offered
by these ISRs are compatible with the Voltage ID
specifications used by popular microprocessors. See
Figure 4-1 for the connection schematic, and the
respective device data sheet for the programming code
information.
Notes:
1. The programming convention is as follows:-
Logic 0: Connect to pin 12; Sense(–).
Logic 1: Open circuit/open drain (See Note 2)
2. For optimal output voltage accuracy Sense(–) (pin 12)
should always be used as the logic ‘0’ reference. The
input/output ground (pins 13-19) can also be used if
pin 12 is connected to pin 13 external to the module.
3. Do not connect pull-up resistors to the voltage
programming pins.
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 IRLML2402 are examples of appropriate
devices.
Active Voltage Programming:
Special precautions should be taken when making changes
to the output voltage progam code while the unit is
powered. This activity can induce current transients
through the device as a result of the electrolytic output
capacitors being either charged or discharged 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 recom-
mended. Making non-incremental changes to VID3 is
discouraged.
When active devices are used to program the output
voltage, their state should be asserted prior to the input
power being applied. One approach is to pull STBY (pin 5)
control to GND prior to power up. After the host board
has then asserted the program code, the ground signal
to pin 5 can be removed. This will allow the regulator to
intiate a clean soft-start power-up to the desired program
voltage. For more information on the use of the Standby
function, consult the related application note, “Using
the Standby Function on the PT7680 Series of 30-A
Switching Regulators.”
Figure 4-1
+5V
STBY
COM
L₁
1µH
(Optional)
4321
VID3 - VID0
7–11
V_IN
PT7681
26
SNS(+)
20–25
V_OUT
STBY
5
GND
13–19
SNS(–)
12
V_OUT
+
C_in
Q₁
L
+
C_out
O
A
D
Sense(–)
Power GND
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