RT9907 Datasheet, PDF (17/19 Pages) Richtek Technology Corporation – 3 Channel DC/DC Converters IC with High-Efficiency Step-Up and Step-Down

Preliminary
RT9907
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.
TSS = 10 x ^CSS (ms)
1nF
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 NMOS 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 NMOS 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 PMOS 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.
DS9907-07 August 2007
www.richtek.com
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