RT9903GQV RICHTEK [Richtek Technology Corporation], RT9903GQV Datasheet - Page 12

• Manufacturer Part Number: RT9903GQV
• Description: 5 Channel DC/DC Converters IC with High-Efficiency Step-Up
• Manufacturer: RICHTEK [Richtek Technology Corporation]

RT9903
Application Information
The RT9903 is a five-Channel DC/DC converter for digital
still cameras and other hand-held device. The five
channels DC/DC converters are as follows:
CH1: Step-up, asynchronous voltage mode DC/DC
converter with an internal power MOSFET, current limit
protection, and over voltage protection. This channel is
designed to supply output voltage from 3.3V to 17V.
CH2: Step-up, asynchronous voltage mode DC/DC
converter with an internal power MOSFET, current limit
protection, and over voltage protection. This channel is
designed to supply output voltage from 3.3V to 17V. At
the same time, it supplies the power for charge pump of
CH5.
CH3: Step-up, asynchronous current mode DC/DC
converter with an internal power MOSFET, current limit
protection, and over voltage protection. This channel is
designed to light 2~4 WLEDs with constant current
regulation, and the lightness can be dimming-controlled
by the duty of EN3 pin.
CH4: Step-up, asynchronous current mode DC/DC
converter with current limit protection. This channel is
designed to drive external N-MOS switch for stepping-
up voltage.
CH5: Charge-pump, to supply negative voltage. This
channel is enabled at the same time as CH2.
Soft-Start
CH1, CH2, and CH4 can be soft-started individually every
time when the channel is enabled. Soft-start is achieved
by ramping up the PWM duty from very small to normal
operation. The ramping up PWM duty is achieved by
sourcing 1uA from error amplifier to the compensation
capacitor. When the output voltage is regulated, the PWM
duty enters the normal operation, and the error amplifier
can sink and source up to 22uA.
The soft-start time is set by the following formula:
R
www.richtek.com
12
COMP
and C
T
SOFT-START
COMP
are compensation components.
=
(1V - 1uA x R
1uA
COMP
) x C
COMP
Preliminary
Oscillator
The internal oscillator synchronizes CH1, CH2, CH3 and
CH4 PWM operation frequency. The operation frequency
is set by a resistor between RT pin to ground, ranging
from 500kHz to 1.4MHz.
Step-up (Boost) DC/DC Converter (CH1)
The channel (CH1) is a step-up voltage-mode DC/DC PWM
converter with built-in internal power MOS and external
schottky diode. Output voltage is regulated and adjustable
up to 17V. This channel is designed to supply several
tens mA current.
The maximum duty of the constant frequency is 96% for
this channel to prevent high input current drawn from
input.
Protection
Current Limit
The current of NMOS is sensed cycle by cycle to prevent
over current. When over current limit, then the NMOS is
off. This state is latched and then reset automatically at
next clock cycle.
Over Voltage
The over voltage protection prevents LX1 voltage going
too high. The over-voltage is detected by the junction
leakage and the threshold value is around 22V. This
channel is latched shut down when OVP occurs, and can
be reset by toggling EN1.
Step-up (Boost) DC/DC Converter (CH2)
The channel (CH2) is a step-up voltage-mode DC/DC PWM
converter with built-in internal power MOS and external
schottky diode. Output voltage is regulated and adjustable
up to 17V. This channel is designed to supply several
tens mA current.
The output voltage of this channel supplies the power of
charge-pump of CH5.
The maximum duty of the constant frequency is 96% for
this channel to prevent high input current drawn from
input.
DS9903-07 August 2007