RT8841 datasheet(28/32 Pages) RICHTEK

28 / 32 page

RT8841

DS8841-01 April 2011

www.richtek.com

If the initial current distribution is constant ratio type,

according to Equation(8), reduce RCSN[1] can reduce IL[1]

and improve current balance. If the initial current distribution

is constant difference type, according to Equation(7),

increase RCSP[1] can reduce IL[1] and improve current

balance.

CMP

ADJ

IMAX

VCC5

OCP

Figure 11. Over Current Protection

In Figure 11, VIMAX is equal to 5V x R2/(R1 + R2). In

RT8841, VADJ is proportional to IOUT and is thermally

compensated. Once VADJ is larger than VIMAX, OCP is

triggered and latched. RT8841 will turn off both high side

MOSFET and low side MOSFET of all channels. A 20uS

delay is used in OCP detection circuit to prevent false

trigger.

Over Voltage Protectiom (OVP)

The over voltage protection monitors the output voltage

via the FB pin. Once VFB exceeds “VEAP + 150mV”, OVP

is triggered and latched. RT8841 will try to turn on low

side MOSFET and turn off high side MOSFET to protect

CPU. A 20

μs delay is used in OVP detection circuit to

prevent false trigger.

COMP

IERR[1] x RCB

IERR[n] x RCB

Interleaved

RAMP[1]

RAMP[n]

CMP

BUF

PWM[1]

PWM[n]

Figure 9. Circuit for Channel Current Balance

In Figure 8, IX[n] is the current signal which is proportional

to current flowing through channel n. In Figure 9, the

current error signals IERR[n] (= IX[n]

− AVG(IX[n])) are used

to raise or lower the internal sawtooth waveforms

(RAMP[1] to RAMP[n]) which are compared with ErrorAmp

output (COMP) to generate PWM signal. The raised

sawtooth waveform will decrease the PWM duty of the

corresponding channel while the lowered will increase.

Eventually, current flowing through each channel will be

balanced.

Channel Current Adjust

If channel current is not balanced due to asymmetric PCB

layout of power stage, external resistors can be adjusted

to correct current imbalance. Figure 10 shows two types

of current imbalance, constant ratio type and constant

difference type.

Constant ratio

IOUT, total

Constant difference

IOUT, total

Figure 10. Channel Current vs. Total Current

Current Balance

Over Current Protection (OCP)

Loop Compensation

The RT8841 is a synchronous Buck converter with two

control loops : voltage loop and current balance loop. Since

the function of the current balance loop is to maintain the

current balance between each active channel, its influence

to converter stability will be negligible compared with the

voltage feedback loop. Therefore, to compensate the

voltage loop will be the main task to maintain converter

stability.

The converter duty-to-output transfer function Gd is :

OUT

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RT8841 Datasheet(PDF) 28 Page - Richtek Technology Corporation

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