RT9907 datasheet(17/19 Pages) RICHTEK

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RT9907

DS9907-11 May 2014

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Application Information

The RT9907 is a three-channel DC/DC converter with one

voltage detector for digital still cameras and other hand-

held device. The three channels DC/DC converters are as

follows:

CH1: Step-up, asynchronous current mode DC/DC

converter with an internal power MOSFET, current limit

protection and high efficiency control for wide loading

range.

CH2: Step-down, synchronous current mode DC/DC

converter with internal power MOSFETs, current limit,

short-circuit , over voltage protection and high efficiency

control for wide loading range.

CH3: Step-down, synchronous current mode DC/DC

converter with internal power MOSFETs, current limit,

short-circuit protection and high efficiency control for wide

loading range.

Soft-Start

CH1, CH2 and CH3 can be soft-started individually every

time when the channel is enabled. Soft-start is achieved

by ramping up the voltage reference of each channel's input

of error amplifier. Adding a capacitor on SS pin to ground

sets the ramping up speed of each voltage reference.

Triangle wave will be appeared on SS pin, which provides

a clock base for soft-start.

The soft-start timing would be setted by following formular.

Oscillator

The internal oscillator synchronizes CH1, CH2 and CH3

PWM operation frequency. The operation frequency is

set by a resistor between RT pin to ground, ranging from

550kHz to 1.4MHz.

Step-up (Boost) DC/DC Converter (CH1)

The step-up channel (CH1) is designed as current-mode

DC/DC PWM converters with built-in internal power MOS

and external Schottky diode. Output voltage is regulated

and adjustable up to 5.5V. This channel typically supplies

3.3V for main system power.

At light load, efficiency is enhanced by pulse-skipping

mode. In this mode, the N-MOSFET turns on by a constant

pulse width. As loading increased, the converter operates

at constant frequency PWM mode. The max. duty of the

constant frequency is 80% for the boost to prevent high

input current drawn from input.

Protection

Current limit

The current of NMOS is sensed cycle by cycle to prevent

over current. If the current is higher than 2.6A (typical),

then the NMOS is off . This state is latched and then

reset automatically at next clock cycle.

Under Voltage

The status of under voltage is decided by comparing FB1

voltage with 0.4V. This function is enabled after soft start

finishes. If the FB1 voltage is less than 0.4V, then the

NMOS will be turned off immediately. And this state is

latched. After a dummy count period, the controller begins

a re-soft-start procedure.

If the status of under voltage remains after 4 successive

times of soft-start, then CH1 is latched.

Over Voltage

The over voltage protection is used when the output of

CH1 supplies the power of the main chip. If the output

voltage of CH1 is over 6.5V, the main chip is shutdown

and the N-MOSFET is kept off.

Step-Down (Buck) DC/DC Converter (CH2, CH3)

The step-down channels (CH2, CH3) are designed as

synchronous current-mode DC/DC PWM converters.

Output voltage is regulated and adjustable down to 0.8V.

The internal synchronous power switches eliminate the

typical Schottky free wheeling diode and improve

efficiency.

At light load, efficiency is enhanced by pulse-skipping

mode. In this mode, the high-side P-MOSFET turns on

by a constant pulse width. As loading increased, the

converter operates at constant frequency PWM mode.

While the input voltage is close to output voltage, the

converter enters low dropout mode. Duty could be as long

as 100% to extend battery life.

(ms)

1nF



RT9907 Datasheet(PDF) 17 Page - Richtek Technology Corporation

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