R2J20751NP

Preliminary

R07DS0240EJ0100 Rev.1.00

Page 21 of 25

Jan 26, 2011

Phase Control

The device incorporates a comparator for control of the phase number. Pulling the voltage on the FB pin up to that on

VCIN exchanges the phase control comparator for the error amplifier, and the device operates as a slave. In this case,

the output of the comparator (CO) is exchanged for the output of the error amplifier (EO), and the positive input

(REFIN) of the error amplifier is exchanged for the positive input (POS) for the comparator. Furthermore, the inverse

input for the comparator is internally connected to the Share pin. The level where the phase number is switched is set by

an external resistor.

Design example;

Specification: L = 470 nH, Fsw = 500 kHz, Vin = 5 V, Vout = 1.5 V, RCS = 820

,

Phase switching level is Iout = 10 A, hysteresis = 3.48 A

1. Deriving the voltage on the Share pins to return to single-phase operation with Iout = 6.52 A (3.48 A of hysteresis)

from two-phase operation with Iout = 10 A.

The peak of the output ripple current is:

Ipp (10 A) = (VIN – Vout) / L

 Vout / VIN / Fsw / 2 + Iout (10 A) = 12.23 A

When Iout = 6.52 A in two-phase mode, current from each device is 3.26 A. Thus,

Ipp (3.26 A) = (VIN – Vout) / L

 Vout / VIN / Fsw / 2 + Iout (3.26 A) = 5.49 A

The ratio between currents for the sense MOS FET and main MOS FET is 1:13700, so a bias current of 300

A

flows through the CS pin.

Thus, voltages on the CS pin are:

Vcs (10 A) = (Ipp (10 A) / 13700 + 300

A)  Rcs = 978 mV and

Vcs (3.26 A) = (Ipp (3.26 A) / 13700 + 300

A)  Rcs = 575 mV.

The non-inverted input terminal of the internal current sense comparator has an offset voltage of 0.2 V, and 40-k



and 10-k

 resistors are connected to the inverted input terminal.

Therefore, the Share voltages are:

Vshare (10 A) = (Vcs (10 A) + 0.2 V)

 5 / 4 = 1.473 V and ......(11)

Vshare (3.26 A) = (Vcs (3.26 A) + 0.2 V)

 5 / 4 = 0.969 V. ......(12)

R4

R3

VCIN

Share

REFIN/POS

EO/CO

R5

Phase control

comparator

Slave chip

EO/CO

Share

Sense

current

0.2V

Master chip

RCS

10k

40k

35k

Current Sense

comparator

Figure 6 Phase Switching Control