RT8805C datasheet(11/18 Pages) RICHTEK

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RT8805C

Preliminary

DS8805C-03 August 2007

www.richtek.com

Figure 3. Current Sensing Loop

The sensing circuit gets IX = IL(S/H) x RDS(ON) x GM by

local feedback. IX is sampled and held just before low side

MOSFET turns off (See Figure 4). Therefore,

IX(S/H) = IL(S/H) x RDS(ON) x GM

Figure 4. Inductor Current and Gate signals

200kHz

DS(ON)

OUT

L(AVG)

(S/H)

OUT

OFF

OUT

L(AVG)

L(S/H)

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

−

⎥⎦

⎤

⎢⎣

⎡

−

High Side MOSFET Gate Signal

Low Side MOSFET Gate Signal

Inductor Current

Falling Slope = V

OUT/L

L(AVG)

L(S/H)

Gate control

a. Before SS signal reach the valley of the ramp voltage,

UGATE and LGATE will be off.

b. If SS pin is pulled down 0.4V, UGATE and LGATE will

be off.

c. UV protect function caused by FB < 0.6V and SS >

3.7V, and controller will trigger Always Hiccup Mode.

d. When OC function occurs and SS > 3.7V, a constant

current of 10

μA starts to discharge the capacitor

connected to SS pin right away. When OC occurs,

UGATE and LGATE will be off. When the voltage at the

capacitor connected to SS pin pass about 0.4V, a

constant current of 10

μA starts to charge the capacitor.

The PWM signal is enable to pass to UGATE and

Current Balance

RT8805C senses the voltage drop of the low-side MOS

and translates this to control the ramp signal. We can see

that the voltage signal finally injected to channel one is

proportional to (IL1 - IL2). Channel two is proportional to

(IL2 - IL1). In steady state and current balance situation,

there is no sensed signal injected into the ramp.

If IL1 > IL2, the ramp bottom of channel 1 will be lifted up

and decreased the duty of UGATE1. On the other hand,

the ramp bottom of channel 2 will be pulled low to increase

the duty of UGATE2. Finally, the loop will be back to the

balance state through above mentioned negative feedback

scheme. Figure 5 shows this scheme.

Figure 5. Current Balance

Logic

Driver

k2 = k x R

ON2

IN2

Logic

Driver

k1 = k x R

ON1

IN1

OUT

CSO2 = k2 x IL2 = k x VON2

CSO1 = k1 x IL1

= k x V

ON1

−

RAMP2

RAMP1

REF

COMP

RAMP2

RAMP1

Current

Balance

OCP

S/H

MUX

PHASE1

PHASE2

CLK2

CLK1

IMAX

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