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DS8088A-01 August 2017 www.richtek.com
RT8088A
©
Copyright 2017 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Operation
The RT8088A is a low voltage synchronous step-down
converter that can support the input voltage range from
2.5V to 5.5V and the output current can be up to 3A. The
RT8088A uses a constant on-time, current mode
architecture. In steady-state operation, the high-side P-
MOSFET is turned on when the current feedback reaches
COMP level which is the amplified difference between the
reference voltage and the feedback voltage. The on-time
of high-side P-MOSFET is determined by on-time
generator which is a function of input and output voltage.
After on-time expires, high-side MOSFET is turned off
and low-side MOSFET is turned on. Until the low-side
current sensing signal reaches the COMP, the high-side
MOSFET is turned on again. In this manner, the converter
regulates the output voltage and keeps the frequency
constant.
The RT8088A reduces the external component count by
integrating the boot recharge MOSFET.
The error amplifier EA adjusts COMP voltage by comparing
the output voltage with the internal I
2
C set reference voltage.
When the load increases, it causes a drop in the output
relative to the reference, then the COMP voltage rises to
allow higher inductor current to match the load current.
PWM Frequency and Adaptive On-Time Control
The on-time can be roughly estimated by the equation :
where f
SW
is nominal 3MHz
Auto-Zero Current Detector
The auto-zero current detector circuit senses the LX
waveform to adjust the zero current threshold voltage.
When the current of low-side MOSFET decreases to the
zero current threshold, the low-side MOSFET turns off to
prevent negative inductor current. In this way, the zero
current threshold can adjust for different condition to get
better efficiency.
Under-Voltage Lockout (UVLO)
The UVLO continuously monitors the VCC voltage to make
sure the device works properly. When the VCC is high
enough to reach the UVLO high threshold voltage, the
step-down converter softly starts or pre-bias to its regulated
output voltage. When the VCC decreases to its UVLO
low threshold voltage, the device will shut down.
Power Good
When the output voltage is higher than PGOOD rising
threshold, the PGOOD flag is high.
Output Under-Voltage Protection (UVP)
When the output voltage is lower than 0.4V after soft-
start, the UVP is triggered. The system will be latched
and the output voltage will no longer be regulated during
UVP latched state. Re-start input voltage or EN pin can
unlatch the protection state. Using I
2
C to shutdown the
system and then re-enable it will also unlatch UVP function.
Over-Current Protection (OCP)
The RT8088A senses the current signal when the low-
side MOSFET turns on. As a result, The OCP is cycle-
by-cycle limit. If the OCP occurs, the converter holds off
the next on pulse until inductor current drops below the
OCP limit.
Soft-Start
An internal current source charges an internal capacitor
to build the soft-start ramp voltage. The typical soft start
time is 150μs.
Over-Temperature Protection (OTP)
The RT8088A has an over-temperature protection. When
the device triggers the OTP, the system will be latched
and the output voltage will no longer be regulated during
OTP latched state. Re-start input voltage or EN pin can
unlatch the protection state. Using I
2
C to shutdown the
system and then re-enable it will also unlatch UVP function.
OUT
ON
IN SW
V
1
T =
Vf
