RT8841 데이터 시트보기 (PDF) - Richtek Technology

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RT8841

4/3/2/1-Phase PWM Controller for High-Density Power Supply

Richtek Technology 

RT8841

RT8841 provides wide range no-load positive offset for over-

clocking applications. The IOFSP capability can supply from

30uA to 640uA, which means in Equation (5), ROFS can

range from 240kΩ to 10kΩ. Other resistances of ROFS

exceeding this range can also provide no-load positive

offset but cannot be guaranteed by Equation (5).

Load Transient Quick Response

VOUT

CFB

RFB

C2

R1 C1

FB

COMP

IOUT

VOUT

FB

Then the output current of CSA will follow the equation

below :

IX = [IL x DCR − VOFS-CSA + 235n x (RCSP − RCSN)]/RCSN

235nA is typical value of CSA input offset current.

VOFS-CSA is the input offset voltage of CSA. VOFS-CSA of

RT8841 is smaller than +/- 1mV. Usually, “VOFS-CSA +

235n x (RCSP − RCSN)” is negligible except at very light

load and the equation can be simplified as the equation

below :

IX = IL x DCR/RCSN

-

EAP = VQR - VADJ

+

-

FB

+

QR

= VEAP

= VEAP - VQR

EAP - VQR

QR

CSA: Current Sense Amplifier

Figure 7. Load Transient Quick Response

IX

In steady state, the voltage of VFB is controlled to be very

235nA

VOFS_CSA

235nA

+

+

-

-

L DCR

RS

CS

ISP

RCSP

ISN

RCSN

close to VEAP. While a load step transient from light load

to heavy load could cause VFB lower than VEAP by several

tens of mV. In prior design, owing to limited control

bandwidth, controller is hard to prevent VOUT undershoot

Figure 8. Circuit for Channel Current Sensing

during quick load transient from light load to heavy load.

RT8841 detects load transient by comparing VFB and VEAP.

If VFB suddenly drops below “VEAP − VQR”, VQR is a

predetermined voltage. The quick response indicator QR

rises up. When QR = H, RT8841 turns on all high side

MOSFETs and turn off all low side MOSFETs. The

sensitivity of quick response can be adjusted by the values

of CFB and RFB. Smaller RFB and/or larger CFB will make

QR easier to be triggered. Figure 7 is the circuit and typical

waveforms.

Output Current Sensing

The RT8841 provides low input offset current-sense

amplifier (CSA) to monitor the output current of every

Loadline

Output current of CSA is summed and averaged in

RT8841. Then 0.5Σ(IX[n]) is sent to ADJ pin. Because

ΣIX[n] is a PTC (Positive Temperature Coefficient) current,

an NTC (Negative Temperature Coefficient) resistor is

needed to connect ADJ pin to GND. If the NTC resistor is

properly selected to compensate the temperature

coefficient of IX[n], the voltage on ADJ pin will be

proportional to IOUT without temperature effect. In RT8841,

the positive input of ErrorAmp is “VDAC − VADJ”. VOUT will

follow “VDAC − VADJ”, too. Thus, the output voltage

decreasing linearly with IOUT is obtained. The loadline is

defined as

channel. Output current of CSA (IX[n]) is used for channel

current balance and active voltage position. In this inductor

current sensing topology, RS and CS must be set according

to the equation below :

L/DCR = RS x CS

LL(loadline) = ΔVOUT/ΔIOUT = ΔVADJ/ΔIOUT

= 0.5 x DCR x RADJ/RCSN

Briefly, the resistance of RADJ sets the resistance of

loadline. The temperature coefficient of RADJ compensates

the temperature effect of loadline.

Copyright ©2014 Richtek Technology Corporation. All rights reserved.

DS8841-02 August 2014

is a registered trademark of Richtek Technology Corporation.

www.richtek.com

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