RT7258
Function Block Diagram
VIN
VCC
V
CC
Internal
Regulator
OSC
Slope
Current Sense
Compensator Amplifier
+
-
Enable
Comparator
+
1.7V
EN/SYNC
5k
3V
-
R
SENSE
Foldback
Control
OTP
V
CC
BOOT
UV Comparator
V
CC
0.4V
+
-
Switch
Controller
Current
Signal
COMP
+
-
45m
SW
0.808V
V
SS
54pF
FB
+
+
EA
-
Current
Comparator
BG
Driver
BG
300k
1pF
PGOOD
Comparator
0.75V
+
-
PGOOD
GND
Operation
The RT7258 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 8A. The RT7258 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
Copyright
©
2012 Richtek Technology Corporation. All rights reserved.
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 : This function is available for
RT7258GQW only. 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.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS7258-00 January 2012
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
