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RT8805C Datasheet PDF : 18 Pages
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RT8805C
Preliminary
Applications Information
Power On Reset
RT8805C operates with input voltage at VCC pin ranging
from 5.9V to 15V. An internal linear regulator regulates
the input voltage to 5V for internal control circuit use. The
POR (power on reset) circuitry monitors the supply voltage
to make sure the supply voltage is high enough for
RT8805C normal work. When the regulated power
exceeds 4.2V typically, the RT8805C releases the reset
state and works according to the setting. Once the
regulated voltage is lower than 4.0V, POR circuitry resets
the chip. Hysteresis between the rising and falling
thresholds assure that once enabled, the RT8805C will
not inadvertently turn off unless the bias voltage drops
substantially (see Electrical Specifications).
Enable, Soft Start and Power Good
Once POR releases, the RT8805C begins its soft start
cycle as shown in Figure 1. A 10μA source current charges
the capacitor CSS connected to SS to control the soft start
behavior of RT8805C. During soft start, SS voltage
increases linearly and clamps the error amplifier output.
Duty cycle and output voltage increase accordingly. The
soft start limits inrush current from input capacitors.
The RT8805C regards SS pin voltage higher than 3.7V as
the end of soft start cycle. Then RT8805C trip PGOOD to
high impedance if no fault occurs indicating power good.
The SS pin also act as the timer during OCP and UVP
hiccup as described in the later sections.
VDD
POR
SS
SSH
FB
PGOOD
> 3.7V
> 0.6V
Figure 1. Power Sequence
Frequency setting
The converter switching frequency is programmed by
connecting a resistor from the RT pin to GND. Figure 2
illustrates switching frequency vs. RRT.
Switching Frequency vs. RT Resistance
1200
1000
800
600
400
200
0
0
20
40
60
80
100
RT Resistance (kΩ)
Figure 2. Switching Frequency vs. RRT.
Voltage Control
The voltage control loop consists of error amplifier,
multiphase pulse width modulator, drivers and power
components. As conventional voltage mode PWM
controller, the output voltage is locked at the positive input
of error amplifier and the error signal is used as the control
signal of pulse width modulator. The PWM signals of
different channels are generated by comparison of EA
output and split-phase sawtooth wave. Power stage
transforms VIN to output by PWM signal on-time ratio.
Current Sensing Setting
RT8805C senses the current of low side MOSFET in each
synchronous rectifier when it is conducting for channel
current balance and OCP detecting. The multiplexer and
sensing GM amplifier converts the voltage on the sense
component (can be a sense resistor or the RDS(ON) of the
low side MOSFET) to current signal into internal circuit
(see Figure 3).
www.richtek.com
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DS8805C-03 August 2007
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