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DS7259-01 November 2013 www.richtek.com
RT7259
©
Copyright 2013 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Function Block Diagram
Operation
The RT7259 is a synchronous high voltage Buck Converter
that can support the input voltage range from 4.5V to 24V
and the output current can be up to 10A. The RT7259
uses a constant frequency, current mode architecture. In
normal operation, the high side N-MOSFET is turned on
when the Switch Controller is set by the oscillator (OSC)
and is turned off when the current comparator resets the
Switch Controller. While the N-MOSFET is turned off, the
external low side N-MOSFET is turned on by BG Driver
with 5V driving voltage from Internal Regulator (V
CC
) until
next cycle begins.
High side MOSFET peak current is measured by internal
R
SENSE
. The Current Signal is where Slope Compensator
works together with sensing voltage of R
SENSE
. The error
amplifier EA adjusts COMP voltage by comparing the
feedback signal (V
FB
) from the output voltage with the
internal 0.808V reference. When the load current
increases, it causes a drop in the feedback voltage relative
to the reference, the COMP voltage then rises to allow
higher inductor current to match the load current.
UV Comparator : If the feedback voltage (V
FB
) is lower
than threshold voltage 0.4V, the UV Comparator's output
will go high and the Switch Controller will turn off the high
side MOSFET. The output under voltage protection is
designed to operate in Hiccup mode.
Oscillator (OSC) : The internal oscillator runs at nominal
frequency 600kHz and can be synchronized by an external
clock in the range between 300kHz and 1.5MHz from EN/
SYNC pin.
PGOOD Comparator : When the feedback voltage (V
FB
)
is higher than threshold voltage 0.75V, the PGOOD open
drain output will be high impedance.
Enable Comparator : Internal 5kΩ resistor and Zener diode
are used to clamp the input signal to 3V. A 1.7V reference
voltage is for EN logic-high threshold voltage. The EN pin
can be connected to VIN through a 100kΩ resistor for
automatic startup.
Foldback Control : When V
FB
is lower than 0.7V, the
oscillation frequency will be proportional to the feedback
voltage.
Soft-Start (SS) : An internal current source charges an
internal capacitor to build the soft-start ramp voltage (V
SS
).
The V
FB
voltage will track the internal ramp voltage during
soft-start interval. The typical soft-start time is 2ms.
Internal
Regulator
+
-
Enable
Comparator
EN/SYNC
1.7V
5k
3V
VIN
OSC
Foldback
Control
+
-
1pF
54pF
300k
+
-
EA
0.808V
UV Comparator
+
-
+
-
Current
Comparator
0.4V
FB
BOOT
45m
SW
PGOOD
GND
Current Sense
Amplifier
Slope
Compensator
PGOOD
Comparator
BG
Driver
BG
Switch
Controller
+
-
0.75V
VCC
R
SENSE
V
CC
+
OTP
COMP
V
CC
V
SS
V
CC
Current
Signal
