PT7707 Datasheet, PDF (3/4 Pages) Texas Instruments – 18 AMP “BIG-HAMMER” PROGRAMMABLE INTEGRATED SWITCHING REGULATOR

Application Notes
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PT7705/7707 Series
More Application Notes
Pin-Coded Output Voltage Adjustment on the
“Big Hammer” Series ISRs
The ISRs related to Power Trends’ PT7705 incorporate pin-
coded voltage control to adjust the ouput voltage. The control
pins are identified VID0 - VID4 (pins 1, 2, 3, 4, & 6) respec-
tively. When the control pins are left open-circuit, the ISR
output 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 selec-
tively grounding VID0 -VID4, the output voltage these ISRs
can be programmed in incremental steps over the specified
output voltage range. In each case, the program code and output
voltage range offered by these ISRs are compatible with the
voltage ID specification defined by Intel Corporation for voltage
regulator modules (VRMs) used to power Pentium® micropro-
cessors. Refer to Figure 1 below for the connection schematic,
and the respective device 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 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.
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
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 ei-
ther charged or discharged to the new output voltage set-point.
The transient current can be minimized by making only incre-
mental 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 result-
ing 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 pro-
grammed voltage.
Figure 1
+5V
STBY
COM
1µH
(Optional)
64321
VID4 - VID0
7-11
Vin
PT7705
Synch
STBY _Out
5 27
GND
13-19
26
SNS(+)
20-25
Vo
SNS(-)
12
+
Cin
+
Cout
L
O
A
D
Power Trends, Inc. 27715 Diehl Road, Warrenville, IL 60555 [phone] Fax: [phone] http://www.powertrends.com
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