RT9248 查看數據表（PDF） - Richtek Technology

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RT9248

Multi-Phase PWM Controller for CPU Core Power Supply

Richtek Technology 

Preliminary

RT9248

Design Example

Given:

Apply for three phase converter

VIN = 12V

VCORE = 1.5V

ILOAD (max) = 60A

VDROOP = 120mV at full load

OCP trip point set at 30A for each channel (S/H)

RDS(ON) = 6mΩ of low side MOSFET at 27°C

L = 2μH

COUT = 9,000μF with 2mΩ ESR.

1. Compensation Setting

a. Modulator Gain, Pole and Zero:

From the following formula:

V IN

12V

Modulator Gain = VRAMP

=

1.9V

×

3

=

4.2

(12.46dB)

2

where VRAMP : ramp amplitude of sawtooth wave

1

LC Filter Pole = 2π x LC = 1.2kHz and

1

ESR Zero = 2π x ESR x COUT = 8.8kHz

b. EA Compensation Network:

Select R1 = 2.4kΩ, R2 = 24kΩ, C1 = 6.6nF,

C2 = 33pF and use the type 2 compensation

scheme shown in Figure 5.

R2 C1

R3 C3

COMP

C2

R1

> VDIF

-

+

DACOUT

FB

R3,C3 are used in type

3 compensation scheme

(left NC in type 2)

Figure 5.

From the following formulas:

FZ =

1

, FP =

1

2π x R2 x C1

Middle Band Gain = R 2

2π x R2 x ⎜⎛ C1× C 2 ⎟⎞

⎝C1 + C2 ⎠

R1

By calculation, the FZ = 1kHz, FP = 200kHz and

Middle Band Gain is 10 (i.e 20dB).

The asymptotic bode plot of EA compensation and

PWM loop gain is shown as Figure 6.

DS9248-05 March 2007

Asymptotic Bode Plot of PWM Loop Gain

100

80

60

Uncompensated EA Gain

40

20

0

Compensated EA Gain

-20

PWM Loop Gain

-40

Modulator Gain

-60

1100

110000

110K00 11000K00 10100000K0 1010M0000 1010000M000

Frequency (Hz)

Figure 6.

2. Droop & DAC Offset Setting

For each channel the load current is 60A / 3 = 20A

and the ripple current, ΔIL, is given as:

5us x 1.5V x ⎜⎛1− 1.5V ⎟⎞ = 3.28A

2uH ⎝ 12V ⎠

The

load

current,

IL,

at

S/H

is 20A

−

ΔIL

2

= 18.36A

.

Using the following formula to select the appropriate

IX (MAX) for the S/H of GM amplifier:

IX (MAX) = R DS(ON) × 18.36A

R SP

The suggested IX is in the order of 40 to 50μA, select

RSP = RSN = 2.4kΩ, then IX (MAX) will be 45.9μA.

VDROOP = 120mV = 45.9μA × 2 × 3 (phase no.) ×

RADJ, therefore RADJ will be 435Ω.

The RDS(ON) of MOSFET varies with temperature

rise. When the low side MOSFET working at 70°C

and 5000ppm/°C temperature coefficient of RDS(ON),

the RDS(ON) at 70°C is given as:

6mΩ × {1+ (70°C − 27°C) × 5000ppm/°C} = 7.3mΩ.

RADJ at 70°C is given as:

RADJ_27°C × (RDS(ON)_27°C / RDS(ON)_70°C) = 358Ω

3. Over-Current Protection Setting

OCP trip point set at 30A for each channel,

IX

=

RDS(ON) × 30A

RSP

= 1.4 × 0.6V ,

RIMAX

RIMAX =

11.2kΩ

Take the temperature rise into account, the RIMAX at

70°C will be:

RIMAX_27°C × (RDS(ON)_27°C / RDS(ON)_70°C) = 9.2kΩ

4. Soft-Start Capacitor Selection

CSS = 0.1μF is the suitable value for most application.

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