RT8113 datasheet(17/21 Pages) RICHTEK

17 / 21 page

RT8113

DS8113-02 April 2011

www.richtek.com

Load Line

The RT8113 utilizes inductor DCR current sense technique

for load line control function. The sensed inductor current

IX is multiplied by 0.5 and sent to ADJ pin. After the current

0.5 x IX injects into the ADJ resistors, the voltage of the

ADJ pin is established. The VADJ is then multiplied by 0.1

and subtracted by VDAC to generate VEAP. Because IX 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,

the voltage on ADJ pin will be proportional to IOUT without

temperature effect. In the RT8113, the positive input of

error amplifier is “VDAC

− 0.1 x VADJ” and VOUT will follow

“VDAC − 0.1 x VADJ”. Thus, the output voltage which

decreases linearly with IOUT is obtained. The load line is

defined as :

ADJ

OUT

ADJ

OUT

CSN

x I x R

DCR x R

LL(Load Line) =

I10

20 x R

ΔΔ

Basically, the resistance of RADJ sets the resistance of

the load line. The temperature coefficient of RADJ

compensates the temperature effect of the load line.

The load line function of the RT8113 can be disabled by

connecting the RT/EN pin resistor to VCC5 instead of

GND. When the RT/EN pin resistor is connected to VCC5,

the current-sense circuit works normally while VEAP no

longer contains droop, and the reference voltage of the

error amplifier will remain equal to VDAC regardless of output

current. The running frequency of the RT8113 will always

be the same whether connecting RT/EN to VCC5 or GND.

Over Current Protection (OCP)

In Figure 8, VOCSET is equal to VCC x R2/(R1 + R2). For

the RT8113, VADJ is proportional to IOUT and is thermally

compensated. Once VADJ is larger than VOCSET, OCP is

triggered and latched. The OCP function will not be

influenced by enabling or disabling load line since the

voltage on the ADJ pin always contains real-time

information of load current. Once OCP is triggered, the

RT8113 will turn off both high-side MOSFETs and low side

MOSFETs.

VCC5 or

VCC12

ADJ

OCSET

OCP

CMP

Figure 8. Over Current Protection

Over Voltage Protection (OVP)

The over-voltage protection monitors the output voltage

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

is triggered and latched. The RT8113 will 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.

Over Temperature Protection (OTP)

The over-temperature protection function of the RT8113 is

built inside the controller to prevent overheat damage. OTP

occurs when the die temperature of the RT8113 exceeds

160

°C, in which the RT8113 then turns off both high-side

MOSFETs and low side MOSFETs.

Loop Compensation

The RT8113 is a voltage mode controller and requires

external compensation. To compensate a typical voltage

mode buck converter, there are two ordinary compensation

schemes, commonly known as type-II compensator and

type-III compensator. The choice of using type-II or type-

III compensator lies with the platform designers, and the

main concern deals with the position of the capacitor ESR

zero and mid-frequency to high-frequency gain boost.

Typically, the ESR zero of output capacitor will tend to

stabilize the effect of output LC double poles. Hence, the

position of the output capacitor ESR zero in frequency

domain may influence the design of voltage loop

compensation. Figure 9 shows a typical control loop using

type-III compensator. Below is the compensator design

procedure.

RT8113 Datasheet(PDF) 17 Page - Richtek Technology Corporation

RT8113 Datasheet(HTML) 17 Page - Richtek Technology Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Datasheet Download

Link URL