PT7750 Datasheet, PDF (3/3 Pages) Texas Instruments – 15 AMP 24V INPUT “BIG-HAMMER III” PROGRAMMABLE ISR

Application Notes
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PT7750 Series
Pin-Coded Output Voltage Adjustment on the
“Big Hammer III” Series ISRs
Power Trends PT7750 series ISRs incorporating pin-coded
voltage control, use pins 1, 2, 3, 4, & 6 to adjust the ouput volt-
age. The control pins are identified VID0 - VID4 respectively.
When the control pins are left open-circuit, the ISR output will
regulate at its factory trimmed output voltage. Each control pin
is internally connected to a precision resistor, and when
grounded increases the output voltage by a set amount. The
internal resistors are binary code weighted, allowing the output
voltage of the ISR to be programmed as a function of a binary
code. VID0 represents the LSB, and VID4 the MSB (or range
change bit). The output voltage ranges offered by these regula-
tors are compatible with some microprocessors, and provide a
convenient method of output voltage selection for many other
applications. Refer to Figure 1 below for the connection sche-
matic, and the PT7750 Data Sheet for the programming 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 program-
ming pins.
3. To minimize output voltage error, always use pin 12 (Re-
mote Sense Ground) as the logic “0” reference. While the
regular ground (pins 13-19) can also be used for program-
ming, doing so will degrade the load reglation of the
product.
More Application Notes
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,
IRLML2402, or the 74C906 hex open-drain buffer 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 in
standby. Changes made to the program code while V_out is en-
abled induces high current transients through the device. This is
the 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 and VID4
with the output enabled is discouraged. If they are changed, the
transients induced can overstress the device resulting in a perma-
nent 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 pro-
grammed voltage.
Figure 1
+24V
STBY
COM
1µH
(Optional)
64321
VID4 - VID0
7-11
Vin
PT7750
Synch
STBY _Out
5 27
GND
13-19
26
SNS(+)
20-25
Vo
SNS(-)
12
+
Cin
+
Cout
L
O
A
D
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