RT8004
Function Block Diagram
RT
ISEN
SYNC
ExtSyn
Oscillator
Slope Com
PVDD
COMP
0.8V
FB
EA
Output
Clamp
OC
Limit
Driver
Ext_SS
Int_SS
LX
EN/SS
SD
0.9V
0.7V
Control
Logic
NISEN
0.4V
NMOS I Limit
0.8V
BG
V
REF
OTP
PGND
PGOOD
POR
POR
VDD
GND
Operation
Main Control Loop
The RT8004 is a monolithic, constant-frequency, current mode step-down DC/DC converter. During normal operation,
the internal top power switch (P-Channel MOSFET) is turned on at the beginning of each clock cycle. Current in the
inductor increases until the peak inductor current reach the value defined by the voltage on the COMP pin. The error
amplifier adjusts the voltage on the COMP pin by comparing the feedback signal from a resistor divider on the FB pin
with an internal 0.8V reference. When the load current increases, it causes a reduction in the feedback voltage relative
to the reference. The error amplifier raises the COMP voltage until the average inductor current matches the new load
current. When the top power MOSFET shuts off, the synchronous power switch (N-Channel MOSFET) turns on until
either the bottom current limit is reached or the beginning of the next clock cycle. The bottom current limit is set at
−2A.
The operating frequency is set by an external resistor connected between the RT pin and ground. The practical switching
frequency can range from 300kHz to 4MHz.
Power Good comparators will pull the PGOOD output low if the output voltage comes out of regulation by 12.5%. In an
overvoltage condition, the top power MOSFET is turned off and the bottom power MOSFET is switched on until either
the overvoltage condition clears or the bottom MOSFET’ s current limit is reached.
Frequency Synchronization
The internal oscillator of the RT8004 can be synchronized to an external clock connected to the SYNC pin. The
frequency of the external clock can be in the range of 300kHz to 4MHz. For this application, the oscillator timing resistor
should be chosen to correspond to a frequency that is about 20% lower than the synchronization frequency.
www.richtek.com
4
DS8004-03
September 2007
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
