RT8805
Applications Information
Power On Reset
Preliminary
Frequency setting
The converter switching frequency is programmed by
connecting a resistor from the RT pin to GND. Figure 2
illustrates switching frequency vs. R
RT
.
RT8805 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 RT8805
normal work. When the regulated power exceeds 4.2V
Switching Frequency vs. RT Resistance
1200
Switching Frequency (kHz)
typically, the RT8805 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 RT8805 will not inadvertently turn off unless
the bias voltage drops substantially (see
Electrical
Specifications).
Enable, Soft Start and Power Good
Once POR releases, the RT8805 begins its soft start cycle
as shown in Figure 1. A 10μA source current charges the
capacitor C
SS
connected to SS to control the soft start
behavior of RT8805. 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 RT8805 regards SS pin voltage higher than 3.7V as
the end of soft start cycle. Then RT8805 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.
1000
800
600
400
200
0
0
20
40
60
80
100
RT Resistance (k )
(kΩ)
Figure 2. Switching Frequency vs. R
RT.
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 V
IN
to output by PWM signal on-time ratio.
Current Sensing Setting
RT8805 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 R
DS(ON)
of the
low side MOSFET) to current signal into internal circuit
(see Figure 3).
VDD
POR
SS
SSH
FB
PGOOD
> 0.6V
> 3.7V
Figure 1. Power Sequence
www.richtek.com
8
DS8805-01
November 2005
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
