ISL8274M datasheet(22/81 Pages) RENESAS

22 / 81 page

FN8931 Rev.1.00

Page 22 of 81

Jan 4, 2018

ISL8274M

4. Functional Description

The output voltage may also be set to any value between 0.6V and 5V using the PMBus command

VOUT_COMMAND. This device supports dynamic voltage scaling by allowing change to the output voltage set

point during regulation. The voltage transition rate is specified by the PMBus command

VOUT_TRANSITION_RATE.

By default, VOUT_MAX is set to 110% of VOUT set by the pin-strap resistor, which can be changed to any value up

to 5.5V by the PMBus Command VOUT_MAX.

4.3

Soft-Start, Stop Delay, and Ramp Times

The ISL8274M follows an internal start-up procedure after power is applied to the VDD pin. The module requires

approximately 60ms to 70ms to check for specific values stored in its internal memory and programmed by

pin-strap resistors. Once this process is completed, the device is ready to accept commands through the PMBus

interface and the module is ready to be enabled. If the module is to be synchronized to an external clock source, the

clock frequency must be stable before asserting the EN pin.

It may be necessary to set a delay from when an enable signal is received until the output voltage starts to ramp to

its target value. In addition, the designer may wish to precisely set the time required for VOUT to ramp to its target

value after the delay period has expired. These features can be used as part of an overall inrush current management

strategy or to precisely control how fast a load IC is turned on. The ISL8274M gives the system designer several

options for precisely and independently controlling both the delay and ramp time periods. The soft-start delay

period begins when the EN pin is asserted and ends when the delay time expires.

The soft-start delay and ramp-up time can be programmed to custom values using the PMBus commands

TON_DELAY and TON_RISE. When the delay time is set to 0ms, the device begins its ramp-up after the internal

circuitry has initialized (approximately 2ms). When the soft-start ramp period is set to 0ms, the output ramps up as

quickly as the output load capacitance and loop settings allow. It is generally recommended to set the soft-start

ramp to a value greater than 2ms to prevent inadvertent fault conditions due to excessive inrush current.

Similar to the soft-start delay and ramp-up time, the delay and ramp down time for soft-stop/off can be

programmed using the PMBus commands TOFF_DELAY and TOFF_FALL. In addition, the module can be

configured as “immediate off” using the command ON_OFF_CONFIG, so that the internal MOSFETs are turned

off immediately after the delay time expires.

The SS/TRACK pin can be used to program the soft-start/stop delay time and ramp time to some typical values as

well as enable/disable the tracking function shown in Table 4 on page 23.

1.4

68.1

1.65

1.8

82.5

1.85

90.9

100

2.4

110

2.5

121

2.8

133

147

3.6

162

178

Table 3. Output Voltage Resistor Settings (Continued)

VOUT1/2 (V)

RSET1/2 (kΩ)

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