PCMB065T-2R2MS datasheet(21/47 Pages) IRF

21 / 47 page

Dual output, 4A/Phase, Highly Integrated SupIRBuck

Single-Input Voltage, Synchronous Buck Regulator

IR3891

www.irf.com

May 08, 2013

up to approximately 3.3V when left floating in normal

operation and only used by channel 2.

In sequencing mode of operation, Vout2 is initially

regulated with the Seq pin. Vout2 ramps up similar to

the normal operation, but Intl_SS is replaced with Seq.

Seq is kept to ground level until Intl_SS signal reaches

its final value. FB2 follows Seq, until Seq approaches

VREF where the E/A switches reference to the VREF.

Vout2 is then regulated with respect to internal VREF

(refer to Figure 17). The final Seq voltage should

between 0.7V and 3.3V.

FB/Vsns

1.3 mS*

PGood

OVP

Is Activated

0.65V

* typical filter delay

Intl_SS

VPG(Upper)

VPG(Lower)

OVP(Threshold)

OVP(Hys)

1.3 mS*

LDrv

turned off

Figure 16: Timing Diagram for Output Sequence

Figure 17: Timing Diagram for Sequence Startup (Seq

ramping up/down)

IR3891 can perform simultaneous or ratiometric

sequencing operations. Simultaneous sequencing is

when the both outputs rise at the same rate. During

Ratiometric sequencing, the ratio of the two outputs is

held constant during power-up.

Figure 19 shows

examples of the two sequencing modes.

IR3891 uses a single configuration to implement both

mode of sequencing operations. Figure 18 shows the

typical circuit configuration for both modes of

sequencing operation. The sequencing mode is

determined by the RA/RB, RE/RF, and RC/RD ratios. If

RE/RF = RC/RD, simultaneous startup is achieved.

Vout2 follows Vout1 until the voltage at the Seq pin

reaches VREF.

After the voltage at the Seq pin

exceeds VREF, VREF dictates Vout2. In ratiometric

startup, Vout2 rises at a slower rate than Vout1. The

resistor values are set up in the following way, RA/RB >

RE/RF > RC/RD.

Table 2 summarizes the required conditions to

achieve simultaneous or ratiometric sequencing

operations.

Table 2: Required Conditions for Simultaneous /

Ratiometric Tracking and Sequencing

Operating Mode

Seq

Required

Condition

Normal

(Non-sequencing,

Non-tracking)

Floating

―

Simultaneous

Sequencing

Ramp up

from 0V

RA/RB>RE/RF=RC/RD

Ratiometric

Sequencing

Ramp up

from 0V

RA/RB>RE/RF>RC/RD

Figure 18: Application Circuit for Simultaneous

and Ratiometric Sequencing

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