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RT2875A/B
3A, 36V, Synchronous Step-Down Converter
General Description
The RT2875A/B is a high efficiency, current-mode
synchronous DC/DC step-down converter that can deliver
up to 3A output current over a wide input voltage range
from 4.5V to 36V. The device integrates 95mΩ high-side
and 70mΩ low-side MOSFETs to achieve high conversion
efficiency. The current-mode control architecture
supports fast transient response and simple external
compensation. A cycle-by-cycle current limit function
provides protection against shorted output and an external
soft-start eliminates input current surge during start-up.
The RT2875A/B provides complete protection functions
such as input under-voltage lockout, output under-voltage
protection, over-current protection and thermal shutdown.
The RT2875A/B is available in the thermal enhanced
TSSOP-14 (Exposed Pad) package.
Features
3A Output Current
Internal N-MOSFETs
Current Mode Control
Adjustable Switching Frequency : 300kHz to 2.1MHz
Adjustable Current Limit : 1.5A to 6A
Synchronous to External Clock : 300kHz to 2.1MHz
Adjustable Output Voltage from 0.6V to 24V
High Efficiency Up to 95%
Stable with Low ESR Ceramic Output Capacitors
Cycle-by-Cycle Current Limit
Input Under-Voltage Lockout
Output Under-Voltage Protection
Thermal Shutdown
AEC-Q100 Grade 2 Certification
RoHS Compliant and Halogen Free
Pin Configurations
(TOP VIEW)
SW
SW
PGND
RT/SYNC
AGND
RLIM
FB
14
2
3
4
5
6
7
15
13
12
Applications
PGND
11
10
9
8
BOOT
VIN
VIN
PGOOD
EN
SS
COMP
Point of Load Regulator in Distributed Power Systems
Digital Set Top Boxes
Broadband Communications
Vehicle Electronics
TSSOP-14 (Exposed Pad)
Simplified Application Circuit
V
IN
C
IN
Enable
PGOOD
VIN
BOOT
C
BOOT
L
SW
EN
PGOOD
RLIM
RT/SYNC
R
LIM
R
OSC
AGND
FB
R
COMP
COMP
SS
C
SS
PGND
C
COMP
R2
R1
C
OUT
V
OUT
RT2875A/B
Copyright
©
2016 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS2875A/B-05 October 2016
www.richtek.com
1
RT2875A/B
Ordering Information
RT2875A/B
Package Type
CP: TSSOP-14 (Exposed Pad)
Lead Plating System
G : Green (Halogen Free and Pb Free)
AQ : Latched UVP
BQ : Hiccup Mode UVP
Note :
Richtek products are :
½
Marking Information
RT2875AQGCP
RT2875AQGCP : Product Number
RT2875AQ
GCPYMDNN
YMDNN : Date Code
RT2875BQGCP
RT2875BQGCP : Product Number
RT2875BQ
GCPYMDNN
YMDNN : Date Code
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.
½
Functional Pin Description
Pin No.
1, 2
Pin Name
SW
Pin Function
Switch Node. Connect to external L-C filter.
Power Ground. The exposed pad must be soldered to a large PCB and
connected to PGND for maximum power dissipation.
Oscillator Resistor and External Frequency Synchronization Input. Must connect
a resistor from this pin to GND to set the switching frequency. If SYNC clock is
requested, connect an external clock to change the switching frequency.
Analog Ground.
Current Limit Setting. Connect a resistor from this pin to GND to set the current
limit value.
Feedback Voltage Input. The pin is used to set the output voltage of the
converter to regulate to the desired via a resistive divider. Feedback reference =
0.6V.
Compensation Node. COMP is used to compensate the regulation control loop.
Connect a series RC network from COMP to GND. In some cases, an additional
capacitor from COMP to GND is required.
Soft-Start Time Setting. Connect a capacitor from SS to GND to set the
soft-start period.
Enable Control Input. High = Enable.
Power Good Indicator Output.
Power Input. Support 4.5V to 36V input voltage. Must bypass with a suitable
large ceramic capacitor at this pin.
Bootstrap Supply for High-Side Gate Driver. Connect a 0.1F ceramic capacitor
between the BOOT and SW pins.
3,
PGND
15 (Exposed Pad)
4
5
6
RT/SYNC
AGND
RLIM
7
FB
8
COMP
9
10
11
12, 13
14
SS
EN
PGOOD
VIN
BOOT
Copyright
©
2016 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
2
DS2875A/B-05 October 2016
RT2875A/B
Function 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.
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
©
2016 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS2875A/B-05 October 2016
www.richtek.com
3
RT2875A/B
UV Comparator
If the feedback voltage is lower than 0.3V, the UV
Comparator will go high to turn off the high-side MOSFET.
The output under voltage protection is designed to operate
in Hiccup mode. When the UV condition is removed, the
converter will resume switching.
Current Setting
The current limit of high side MOSFET is adjustable by
an external resistor connected to the RLIM pin. The current
limit range is from 1.5A to 6A.
Thermal Shutdown
The over-temperature protection function will shut down
the switching operation when the junction temperature
exceeds 180°C. Once the junction temperature cools
down by approximately 15°C, the converter will
automatically resume switching.
Copyright
©
2016 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS2875A/B-05 October 2016
RT2875A/B
Absolute Maximum Ratings
(Note 1)
−0.3V
to 40V
−0.3V
to (V
IN
+ 0.3V)
−0.3V
to 6V
−0.3V
to 40V
−0.3V
to 6V
Supply Voltage, VIN ------------------------------------------------------------------------------------------------
Switch Voltage, SW ------------------------------------------------------------------------------------------------
BOOT to SW ---------------------------------------------------------------------------------------------------------
Power Good Voltage, PGOOD ------------------------------------------------------------------------------------
Other Pins -------------------------------------------------------------------------------------------------------------
Power Dissipation, P
D
@ T
A
= 25°C
TSSOP-14 (Exposed Pad) ---------------------------------------------------------------------------------------- 4.464W
Package Thermal Resistance (Note 2)
TSSOP-14 (Exposed Pad),
θ
JA
----------------------------------------------------------------------------------
TSSOP-14 (Exposed Pad),
θ
JC
----------------------------------------------------------------------------------
Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------------
Junction Temperature -----------------------------------------------------------------------------------------------
28°C/W
4.3°C/W
260°C
150°C
Storage Temperature Range --------------------------------------------------------------------------------------
−65°C
to 150°C
ESD Susceptibility (Note 3)
HBM (Human Body Model) ---------------------------------------------------------------------------------------- 2kV
(Note 4)
Recommended Operating Conditions
Supply Input Voltage, VIN ----------------------------------------------------------------------------------------- 4.5V to 36V
Junction Temperature Range --------------------------------------------------------------------------------------
−40°C
to 150°C
Ambient Temperature Range --------------------------------------------------------------------------------------
−40°C
to 105°C
Electrical Characteristics
(V
IN
= 12V, T
A
=
−40°C
to 105°C, unless otherwise specified)
Parameter
Shutdown Supply Current
Switching quiescent current with
no load at DCDC output
Feedback Voltage
High-Side
Symbol
Test Conditions
V
EN
= 0V
V
EN
= 2V, V
FB
= 0.64V,
R
LIM
= 91k, R
OSC
= 169k
Min
--
--
0.588
--
--
--
Typ
‐‐
--
0.6
950
95
70
1
--
2.1
4
5.5
2
5.2
--
Max
10
1.3
0.612
--
--
--
--
6
2.41
4.48
6.16
--
--
2100
Unit
A
mA
V
A/V
m
A
A
A
A
A
A
A/V
kHz
V
FB
R
DS(ON)1
R
DS(ON)2
4.5V
V
IN
36V
IC
= ±10A
Error Amplifier Trans-conductance G
EA
Switch On-
Resistance
Low-Side
High-Side Switch Leakage
Current
Current Limit Setting Rage
High-Side Switch Current Limit 1
High-Side Switch Current Limit 2
High-Side Switch Current Limit 3
Low-Side Switch Current Limit
COMP to Current Sense
Transconductance
Switching Frequency Range
G
CS
V
EN
= 0V, V
SW
= 0V
(Note 5)
H
OC1
H
OC2
H
OC3
R
LIM
= 100k
R
LIM
= 47k
R
LIM
= 33k
From Drain to Source
--
1.5
1.79
3.52
4.84
--
--
Include Sync mode and RT mode
set point
300
Copyright
©
2016 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS2875A/B-05 October 2016
www.richtek.com
5
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