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NX2415

TWO PHASE SYNCHRONOUS PWM CONTROLLER WITH INTEGRATED FET DRIVER AND DIFFERENTIAL CURRENT SENSE

Microsemi Corporation 

NX2415

Combine equation 22 and 23,

RDROOP

=

0.6

2

60kΩ

2kΩ + RS

DCRILOADRIN

∆VOUT

...(24)

Where DCR is the sense resistor or the DCR of

the output inductor. RS is the current sensing matching

resistor when using DCR sensing method. ILOAD is the

load current. RIN is the input DC resistor of the master

phase compensator which connect FB pin and PGSEN

pin. For example, to have the DVOUT=60mV when the

load

current

is

20A,

DCR

is

1.4mΩ,

R

IN

is

10kΩ,

R

S

is

620Ω.

RDROOP

=

0.6

2

×

60kΩ

2kΩ + 0.62kΩ

× 1.4mΩ × 20A ×10kΩ

60mV

= 32kΩ

Choose

R=

DROOP

32kΩ.

Over Voltage Protection

Over voltage protection is achieved by sensing the

output voltage through resistor divider. The sensed volt-

age on PGSEN pin is compared with 120%*0.8V to gen-

erate the OVP signal. A small value capacitor is re-

quired to connect to PGSEN pin also.

be calculated. From this figure, it is obvious that a multi-

phase converter can have a much smaller input RMS

current, which results in a lower amount of input capaci-

tors that are required.

For example, Vin=12V, Vout=1.2V. The duty cycle

is D=Vout/Vin=1.2/12=10%. From the figure, for two

phase, the normlized RMS current is

0.2*Iout=0.2*50A=10A.

A combination of ceramic and electrolytic(SANYO

WG or WF series) or OSCON type capacitors can

achieve both ripple current capability together with hav-

ing enough capacitance such that input voltage will not

sag too much. In this application, one OSCON

SVPC180M(180uF, 16V, 2.8A) and three 10uF(4A rms

current, X5R) ceramic capacitors are selected.

A 1uH input inductor is recommended to slow down

the input current transient. Suppose power stage effi-

ciency is 0.8, then input current can estimated by

II N P U T

= IOUT × VOUT

η × VIN

=

60A × 1.2V

0.8 × 12V

= 7.5A

In this application, Coilcraft DO3316P_102HC with

RMS rating 10A is chosen.

VOUT 1.2V

10k

0.8V*120%

PGSEN

20k

1nF

0.5

OVP

0.4

( ) IRMS IN 0.3

Iout

0.2

Single-

phase

Two

phase

Figure 14 - Over voltage protection

Input Filter Selection

The selection criteria of input capacitor are voltage

rating and the RMS current rating. For conservative con-

sideration, the capacitor voltage rating should be 1.5

times higher than the maximum input voltage. The RMS

current rating of the input capacitor for multi-phase con-

verter can be estimated from the above Figure 15.

First, determine the duty cycle of the converter (VO/

V ). The ratio of input RMS current over output current

IN

can be obtained. Then the total input RMS current can

0.1

0

0

Three

phase

0.1 0.2 0.3 0.4 0.5

D

Figure 15 - Normalized input RMS current vs.

duty cycle.

Rev.4.8

17

05/06/08

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