RT2875A/B
Functional Block Diagram
PGOOD
VIN
6k
EN
1.5V
3.8V
-
+
Shutdown
Comparator
UVLO
Internal
Regulator
Current
Sense
+
BOOT
UVLO
BOOT
Power
Stage &
Deadtime
Control
SW
0.55V
-
PGOOD
Logic &
Comparator Protection
Control
0.3V
+
UV
Comparator
FB
0.6V
-
+
EA
+
-
HS Switch
Current
Comparator
Oscillator
Slop
Compensation
LS Switch
Current
Comparator
Current
Sense
PGND
AGND
6µA
SS
COMP RT/SYNC
RLIM
Operation
The RT2875A/B is current-mode synchronous step-down
converter. In normal operation, the high-side N-MOSFET
is turned on when the S-R latch is set by the oscillator
and is turned off when the current comparator resets the
S-R latch. While the high-side N-MOSFET is turned off,
the low-side N-MOSFET is turned on to conduct the
inductor current until next cycle begins.
Error Amplifier
The error amplifier adjusts its output voltage by comparing
the feedback signal (V
FB
) with the internal 0.6V reference.
When the load current increases, it causes a drop in the
feedback voltage relative to the reference, and then the
error amplifier's output voltage rises to allow higher inductor
current to match the load current.
Switching Frequency
The switching frequency can be set by using extra resister
RT or external clock. Switching frequency range is from
300kHz to 2.1MHz.
BOOT UVLO
The BOOT UVLO circuit is implemented to ensure a
sufficient voltage of BOOT capacitor for turning on the high-
side MOSFET at any condition. The BOOT UVLO usually
is a registered trademark of Richtek Technology Corporation.
Internal Regulator
The regulator provides low voltage power to supply the
internal control circuits and the bootstrap power for high-
side gate driver.
Enable
The converter is turned on when the EN pin is higher than
1.6V. When the EN pin is lower than 0.4V, the converter
will enter shutdown mode and reduce the supply current
lower than 10μA.
Soft-Start (SS)
In order to prevent the converter output voltage from
overshooting during the startup period, the soft-start
function is necessary. The soft-start time is adjustable
by an external capacitor.
Copyright
©
2018 Richtek Technology Corporation. All rights reserved.
DS2875A/B-08 November 2018
www.richtek.com
3
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
