RT9902 datasheet(20/21 Pages) RICHTEK

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RT9902

Preliminary

DS9902-10 August 2007

www.richtek.com

Protection

Current Limit (CH2, CH3)

The current of high-side PMOS is sensed cycle by cycle

to prevent over current. If the current is higher than 1.5A

(typical), then the high-side PMOS is off and the low-side

NMOS is on. This state is latched and then reset

automatically at next clock cycle.

Under Voltage (CH2, CH3)

The status of under voltage is decided by comparing FB2

(or FB3) voltage with 0.4V. This function is enabled after

soft-start finishes. If the FB2 (or FB3) voltage is less than

0.4V, then the high/low-side power MOS are turned off

immediately. And this state is latched. After a dummy

count period, the CH2 (or CH3) begins a soft-start

procedure.

However, if the status of under voltage remains after 3

successive times of soft-start, then CH2 (or CH3) is

latched.

UV remain after 3

successive soft-start

How to reset?

CH2 CH2 is latched, and whole

IC is shut down

Toggle ENM

CH3 CH3 is latched

Toggle EN3 or ENM

Over Voltage Protection (CH2)

Over voltage protection (OVP) is used to protect the

external parts connected to the output of CH2. If the FB2

voltage is higher than 1V, the high-side PMOS is off and

low-side NMOS is on. This status is latched and could be

reset by toggling ENM.

Charge Pump DC/DC Converter

This is a low quiescent charge pump DC/DC converter,

which is enabled by ENM. Add a capacitor CX (~1nF)

between charge pump VOUT and CPFB to speed up charge

pump response time. Output ripple can be easily

suppressed by increasing the capacitance ratio of COUT

and CCP. This charge pump DC/DC converter can apply

μC stanby power or the gate driver power of IGBT for

photoflash, etc.

The maximum output current can be determined by CCP

and COUT ration. This equation would describe the

relationship.

IMAX = 2 x (VDDC-VF) x CCP x FCP

VF : Schottky diode forward voltage

Fpump : Charge pump maximum frequency is 500kHz

Recommand CCP

≤ 0.1μF.

Reference

The chip has an internal 0.8V reference voltage, which is

the inputs of the error amplifiers of the CH1, CH2, and

CH3 to compare the difference of feedback voltage. The

reference voltage can be set up stably when the supplied

power (VDDM) is above 1.5V, and EN1 (or EN2, EN3)

goes high.

Thermal Protection

Thermal protection function is integrated in the chip. When

the chip temperature is higher than 178

°C, the controllers

of CH1, CH2, and CH3 are shutdown. 10

°C is the

hysteresis range of temperature to prevent unstable

operation when the thermal protection happens. When

the thermal protection is relieved, the chip operates well

again.

VDDC

CPFB

GND

BAT

OUT

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