TPS2306

DUAL SEQUENCING HOT SWAP POWER MANAGER

SLVS368 – APRIL 2001

21

POST OFFICE BOX 655303

APPLICATION INFORMATION

R1B

+

0.65 V

(2.5 V

* 0.65 V)

10k

W ^ 3.51 kW

estimating the output ramp-up time

As described in the Fault Timer section, TPS2306 operation in linear mode is limited to the timeout period

established by the external capacitor on the CT pin. This timeout must be long enough to allow for either the

concurrent or sequential charging of BOTH outputs to the input supply levels. Therefore, an estimate of this

ramp-up time is required to ensure start-up.

The output voltage ramp profiles are influenced by a number of factors, including the amount of load

capacitance, the load current level, the load characteristic (either resistive or constant-current), and the current

ramp profile. Therefore, determination of the charging, or start-up, time is organized into several different load

categories:

1.

resistive loads, no soft-start

2.

constant-current loads, no soft-start

3.

constant-current loads, with soft-start

The case which most closely describes the application characteristics should be selected.

NOTE:In the following discussion, the subscripted “1” and “2” suffixes refer respectively to the first

channel to ramp up (primary or master), and the second (or controlled or slave) channel to ramp,

not necessarily device Channels 1 and 2.

case 1: resistive loads, no soft-start

When the output ramp conditions are met, the pass FET gates are initially only driven to source a current of

approximately 5% of the IMAX current to the loads, as described in the Output Ramp Operation section. This

first step lasts a nominal 500

µs. Subsequently, the output current is ramped to the full IMAX setting. For typical

applications, the extent of capacitance charging during this slow ramp period is relatively small. Therefore, it

can be neglected in estimating total start-up time, simplifying calculations. However, since the HSPM timer is

started as soon as the first output starts ramping, the slow ramp period is included as a component of the total

ramp-up time.

Given these considerations, the start-time calculation approximates the source as making a step change from

from:

t

+ R

Lx

C

Lx

n

(IMAXx)

R

Lx

IMAXx

R

Lx * nx

(t)

) t

SR

where:

(15)

(16)