®
RT7258
8A, 24V, 600kHz Step-Down Converter with Synchronous
Gate Driver
General Description
The RT7258 is a synchronous step-down DC/DC converter
with an integrated high side internal power MOSFET and
a gate driver for a low side external power MOSFET. It
can deliver up to
8A
output current from a 4.5V to 24V
input supply. The RT7258's current mode architecture
allows the transient response to be optimized over a wider
input voltage and load range. Cycle-by-cycle current limit
provides protection against shorted outputs and soft-start
eliminates input current surge during start-up. The RT7258
is synchronizable to an external clock with frequency
ranging from 300kHz to 1.5MHz.
The RT7258 is available in WDFN-14L 4x3 and SOP-8
(Exposed Pad) packages.
Features
4.5V to 24V Input Voltage Range
8A
Output Current
45mΩ Internal High Side N-MOSFET
Ω
Current Mode Control
600kHz Switching
Frequency
Adjustable Output from 0.808V to 15V
Up to 95% Efficiency
Internal Compensation
Stable with Ceramic Capacitors
Synchronous External Clock : 300kHz to 1.5MHz
Cycle-by-Cycle Current Limit
Input Under Voltage Lockout
Output Under Voltage Protection
Power Good Indicator
Thermal Shutdown Protection
RoHS Compliant and Halogen Free
Applications
Point of Load Regulator in Distributed Power System
Digital Set top Boxes
Personal Digital Recorders
Broadband Communications
Flat Panel TVs and Monitors
Simplified Application Circuit
RT7258
BOOT
C
BOOT
SW
VCC
C
VCC
Power Good
Chip Enable
EN/SYNC
GND
PGOOD
FB
R2
BG
Q1
R1
C
OUT
L
V
OUT
V
IN
C
IN
VIN
Copyright
©
2013 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7258-01
November 2013
www.richtek.com
1
RT7258
Ordering Information
RT7258
Package Type
QW : WDFN-14L 4x3 (W-Type)
SP : SOP-8 (Exposed Pad-Option 2)
Lead Plating System
G : Green (Halogen Free and Pb Free)
Note :
Richtek products are :
½
Marking Information
RT7258GQW
13= : Product Code
13=YM
DNN
YMDNN : Date Code
RT7258GSP
RT7258GSP : Product Number
RoHS compliant and compatible with the current require-
ments of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
RT7258
GSPYMDNN
YMDNN : Date Code
½
Pin Configurations
(TOP VIEW)
FB
PGOOD
EN/SYNC
VIN
VIN
VIN
NC
1
2
3
4
5
6
7
14
13
12
GND
15
11
10
9
8
GND
BG
VCC
BOOT
SW
SW
SW
SW
BOOT
VCC
BG
2
3
4
8
VIN
EN/SYNC
FB
GND
GND
9
7
6
5
WDFN-14L 4x3
SOP-8 (Exposed Pad)
Copyright
©
2013 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
2
DS7258-01
November 2013
RT7258
Functional Pin Description
Pin No.
WDFN-14L 4x3
SOP-8
(Exposed Pad)
6
Pin Name
Pin Function
Feedback Input. This pin is connected to the converter output. It
is used to set the output of the converter to regulate to the
desired value via an external resistive divider. The feedback
reference voltage is 0.808V typically.
Power Good Indicator with Open Drain. (for RT7258GQW only)
A 100k pull-high resistor is needed. The output of this pin is
pulled to low when the FB is lower than 0.75V; otherwise it is
high impedance.
Enable or External Frequency Synchronization Input. A
logic-high (2V < EN < 5.5V) enables the converter; a logic-low
forces the IC into shutdown mode reducing the supply current to
less than 3A. For external frequency synchronization operation,
the available frequency range is from 300kHz to 1.5MHz.
Power Input. The available input voltage range is from 4.5V to
24V. A 22F or larger input capacitor is needed to reduce
voltage spikes at the input.
No Internal Connection.
Switching Node. Output of the internal high side MOSFET.
Connect this pin to external low side N-MOSFET, inductor and
bootstrap capacitor.
Bootstrap for High side Gate Driver. Connect a 1F ceramic
capacitor between the BOOT pin and SW pin.
BG Driver Bias Supply. Decouple with a 1F X5R/X7R ceramic
capacitor between the VCC pin and GND.
Gate Driver Output. Connect this pin to the gate of the external
low side N-MOSFET.
Ground. The exposed pad must be soldered to a large PCB and
connected to GND for maximum thermal dissipation.
1
FB
2
--
PGOOD
3
7
EN/SYNC
4, 5, 6
7
8, 9, 10
11
12
13
8
--
1
2
3
4
VIN
NC
SW
BOOT
VCC
BG
14,
5,
GND
15 (Exposed Pad) 9 (Exposed Pad)
Copyright
©
2013 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7258-01
November 2013
www.richtek.com
3
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
©
2013 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
DS7258-01
November 2013
RT7258
Absolute Maximum Ratings
(Note 1)
–0.3V
to 26V
–0.3V
to (V
IN
+ 0.3V)
–5V
to 30V
–0.3V
to 6V
−0.3V
to 6V
1.667W
Supply Input Voltage, V
IN
------------------------------------------------------------------------------------------
Switching Voltage, SW --------------------------------------------------------------------------------------------
SW (AC) < 20ns -----------------------------------------------------------------------------------------------------
BOOT to SW ---------------------------------------------------------------------------------------------------------
All Other Voltage ----------------------------------------------------------------------------------------------------
Power Dissipation, P
D
@ T
A
= 25°C
WDFN-14L 4x3 -------------------------------------------------------------------------------------------------------
SOP-8 (Exposed Pad) --------------------------------------------------------------------------------------------- 1.333W
Package Thermal Resistance (Note 2)
WDFN-14L 4x3,
θ
JA
------------------------------------------------------------------------------------------------- 60°C/W
WDFN-14L 4x3,
θ
JC
------------------------------------------------------------------------------------------------- 7.5°C/W
SOP-8 (Exposed Pad),
θ
JA
---------------------------------------------------------------------------------------- 75°C/W
SOP-8 (Exposed Pad),
θ
JC
--------------------------------------------------------------------------------------- 15°C/W
Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------- 260°C
Junction Temperature ----------------------------------------------------------------------------------------------- 150°C
Storage Temperature Range --------------------------------------------------------------------------------------
–65°C
to 150°C
ESD Susceptibility (Note 3)
HBM (Human Body Mode) ---------------------------------------------------------------------------------------- 2kV
MM (Machine Mode) ------------------------------------------------------------------------------------------------ 200V
(Note 4)
Recommended Operating Conditions
Supply Input Voltage, V
IN
------------------------------------------------------------------------------------------ 4.5V to 24V
Junction Temperature Range --------------------------------------------------------------------------------------
−40°C
to 125°C
Ambient Temperature Range --------------------------------------------------------------------------------------
−40°C
to 85°C
Electrical Characteristics
(V
IN
= 12V, T
A
= 25°C, unless otherwise specified)
Parameter
Shutdown Supply Current
Supply Current
Reference Voltage
Feedback Current
High Side Switch On Resistance
High Side Switch Current Limit
Oscillation Frequency
Short Circuit Oscillation Frequency
Maximum Duty Cycle
Minimum On-Time
Symbol
Test Conditions
V
EN
= 0V
V
EN
= 3V, V
FB
= 1V
Min
--
--
--
--
--
--
--
--
--
4
--
Typ
1
0.9
10
45
13
600
190
90
100
4.2
400
Max
--
--
0.82
--
--
--
--
--
--
--
4.4
--
Unit
A
mA
V
nA
m
A
kHz
kHz
%
ns
V
mV
V
REF
I
FB
R
DS(ON)
4.5V
V
IN
24V
V
FB
= 0.8V
BOOT
SW = 4.8V
0.796 0.808
f
OSC1
f
OSC2
D
MAX
t
ON
V
FB
= 0V
V
FB
= 0.6V
V
FB
= 1V
Input Under Voltage Lockout Threshold V
UVLO
Input Under Voltage Lockout Threshold
V
UVLO
Hysteresis
Copyright
©
2013 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7258-01
November 2013
www.richtek.com
5
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