R2J20701NP

REJ03G1459-0300 Rev.3.00 Jun 20, 2007

Page 13 of 27

Case 4)

Current sharing or two-phase operation

In the case of master–slave operation, the TRK-SS pin on the master device should be attached to an RC

network for soft starts. TRK-SS pins of slave devices should be tied to the master’s TRK-SS pin. The error

amplifiers on the slave devices can be disabled by pulling up the corresponding FB pins to REG5, and the

slave devices do not require loop-compensation networks.

REG5

TRK-SS

Ishare

Ishare

FB

Channel 1

(Master)

SW

R

Vout1

C

R

C

R1

REG5

TRK-SS

FB

CT

CT

Channel 2

(Slave)

SW

R2

Vout (nominal) = 0.6 V

× (R1 + R2) / R2

To SGND (for current sharing and synchronized operation)

To REG5 (for current sharing and two-phase operation)

Choice of The Resistance of CS Pin

The CS pin is a current-output pin. A current 1/18500 of that of the high-side MOS FET flows through this pin, which

also has a dc current offset of 490

µA. The converter’s maximum current is determined by the voltage on the CS pin,

i.e. 1.5 V, and by the value of the external resistor attached to this pin. The resistance is determined as shown below.

Specification: L = 360 nH, Vin = 12 V, Vout = 1.8 V, Fsw = 500 kHz, Iout(max) = 25 A

Current through the choke coil is

ILpp = (Vin – Vout)

× Vout / (L × Vin × Fsw) = 8.5 A (p-p)

Peak choke current is the current when Io is at its maximum, i.e.

Ilmax = Io(max) + 0.5

× ILpp = 25A + 4.25A = 29.25 A

Maximum CS pin output current is;

Icsmax = Ilmax / 18500 + Ics-dc = 29.25 A / 18500 + 490

µA = 2.071 mA

The ideal resistance for attachment to the CS pin is

RCS = Vcl / Icsmax =1.5 V / 2.071 mA = 724

Ω

Therefore choose 750

Ω as the value of the resistor for attachment to the CS pin.