RAA210825 datasheet(21/56 Pages) RENESAS

21 / 56 page

FN9349 Rev.1.00

Page 21 of 56

Feb 8, 2019

RAA210825

4. Functional Description

4.3

Soft-Start Delay and Ramp Times

The RAA210825 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. When this process is complete, the device is ready to accept commands from the PMBus

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

clock frequency must be stable prior to 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 RAA210825 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 (TON_DELAY) and ramp-up time (TON_RISE) can be set to custom values with pin-strap

resistors or PMBus. 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. In general, set the soft-start ramp to a value greater than 500µs

to prevent inadvertent fault conditions due to excessive inrush current.

Similar to the soft-start delay and ramp-up time, the delay (TOFF_DELAY) and ramp-down time (TOFF_FALL)

for soft-stop/off can be set to custom values with pin-strap resistors or PMBus. In addition, the module can be

configured as “immediate off” with the command ON_OFF_CONFIG, such that the internal MOSFETs are turned

off immediately after the delay time expires.

The SS/UVLO pin can program the soft-start/stop delay time and ramp time to some typical values as shown in

Table 6. A standard 1% resistor is required.

Table 6. Soft-Start/Stop Resistor Settings

Resistor (kΩ)

Delay Time (ms)

Ramp Time (ms)

Open

Connect to V25

Connect to SGND

12.1

13.3

14.7

16.2

17.8

19.6

21.5

23.7

26.1

28.7

31.6

34.8

38.3

42.2

46.4

51.1

56.2

RAA210825 Datasheet(PDF) 21 Page - Renesas Technology Corp

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