RT8004A 데이터 시트보기 (PDF) - Richtek Technology
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RT8004A Datasheet PDF : 13 Pages
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RT8004A
Function Block Diagram
RT
SYNC
ExtSyn
Oscillator
Slope Com
ISEN
PVDD
COMP
FB
0.8V
EA
Output
Clamp
EN/SS
Ext_SS
SD
Int_SS
0.9V
0.7V
0.4V
POR
POR
BG
OC
Limit
Driver
Control
Logic
0.8V
VREF
OTP
NISEN
N-MOSFET
I Limit
LX
PGND
PGOOD
Operation
VDD GND
Main Control Loop
The RT8004A is a monolithic, constant-frequency, current mode step-down DC/DC converter. During normal operation,
the internal top power switch (P-MOSFET) is turned on at the beginning of each clock cycle. Current in the inductor
increases until the peak inductor current reaches 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 resistive voltage 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-MOSFET) turns on until either the bottom
current limit is reached or the next clock cycle begins. 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.
Dropout Operation
When the input supply voltage decreases toward the output voltage, the duty cycle increases toward the maximum on-
time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle eventually
reaching 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across
the internal P-MOSFET and the inductor.
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DS8004A-03 March 2011
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