RT2702 datasheet(15/20 Pages) RICHTEK

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RT2702

DS2702-00 October 2016

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

ILOAD

IL,VALLEY

tON

IL,PEAK

Figure 4. “Valley” Current Limit

Current Limit Threshold Setting

Current limit threshold can be set by two resistors, one is

RILIM between ILIM and GND, and other is RCS of the DCR

circuit, as shown in Figure 5. Once VIN exceeds the POR

threshold and chip is enabled, an internal voltage source

1.2V will be generated, and then the current IILIM will flow

through RILIM. In Figure 5, IILIM is mirrored to flow through

RCS of the DCR circuit by current mirror, and then the

voltage VCS will be generated. With the DCR circuit, the

CSEN voltage VCSEN is inductor current IL multiplied by

inductor DCR RL,DCR. By using an internal comparator in

the RT2702, when VCSEN valley is higher than the current

limit protection threshold VCS, the PWM is not allowed to

initiate a new cycle.

Therefore, RILIM can be determined using the following

equation :

ILIM

L,VALLEY

L,DCR

1.2 R







where IL,VALLEYrepresents the desired inductor limit current

(valley inductor current).

Besides, in order to ensure the IL waveform can be reflected

on VCSEN, the time constant setting should follow the

equation below :

SEN

L,DCR



VOUT

PHASE

1.2V

RCS

RSEN

CSEN

RL,DCR

CSP

CSN

ILIM

+ VCS -

VCSEN

Comp

Current Limit

IILIM

RILIM

IILIM

Figure 5. External Current Limit Setting

Thermal Compensation for Current Sense

Since the copper wire of inductor has a positive

temperature coefficient. And hence, temperature

compensation is necessary for the lossless inductor

current sense. Figure 6 shows the thermal compensation

method for current sense circuit that is not only simple

but also effective way to compensate temperature variation.

An NTC thermistor is employed in the thermal

compensation network, and it can be used to compensate

DCR variation when temperature is changed. Usually, RP

is set equal to RNTC(25

°C). RS is selected to linearize the

NTC's temperature characteristic. For a given RNTC, RSEN

and RS can be obtained to compensate the temperature

variation of the sense resistor. Besides, the optional

CBYPASS is used to reduce noise to affect the current sense

network.

Figure 6. Thermal Compensation Method

RNTC

CSEN

VCSEN -

RSEN

RL,DCR

CBYPASS

VOUT

PHASE

RCS

VCS -

CSP

CSN

Thermal compensation

method

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