3

Applicatio n

Note s

PT7720 Series

More Application Notes

Pin-Coded Output Voltage Adjustment on the

“Big Hammer II” Series ISRs

Power Trends PT7720 series ISRs incorporate pin-coded volt-

age 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 output will

regulate at its factory trimmed output voltage. Each pin is inter-

nally 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 specified output voltage range. The output voltage

ranges offered by these regulators provide a convenient method

of output voltage selection for many applications. In addition,

the program code and output voltage range of the PT7721

model ISR is compatible with the voltage ID specification de-

fined by Intel Corporation for voltage regulator modules

(VRMs) used to power Pentium® microprocessors. Refer to

Figure 1 below for the connection schematic, and the PT7720

Data Sheet for the appropriate 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-

Figure 1

ming, doing so will degrade the load reglation of the prod-

uct.

4. If active devices are used to ground the voltage control pins,

low-level open drain MOSFET devices should be used over

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

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

the device output to execute a soft-start power-up to the pro-

grammed voltage.

Cout

+

Cin

+

1

µH

(Optional)

+12V

COM

STBY

L

O

A

D

PT7720

6

4

3

2

1

26

20-25

12

5

13-19

27

7-11

Vo

Vin

GND

SNS(+)

SNS(-)

Synch

Out

STBY

VID4 - VID0