RT5735
5A Step-Down Converter with I
2
C Interface
General Description
The RT5735 device is a full featured 5.5V, 5A,
synchronous step down constant-on-time (COT)
current mode converter with two integrated MOSFETs.
The RT5735 enables small designs by integrating the
MOSFETs, implementing current mode control to
reduce external component count, reducing inductor
size by enabling up to 2.4MHz switching frequency,
and minimizing the IC footprint with a small
WL-CSP-20B 1.6x2 (BSC) package. The RT5735
provides accurate regulation for a variety of loads with
an accurate ±1% Voltage Reference (VREF) over
temperature. Efficiency is maximized through the
integrated 34m/18m MOSFETs and 70A typical
quiescent current. Using the enable pin, shutdown
supply current is less than 2A by entering a shutdown
mode.
The output voltage startup ramp is controlled by the
slow start pin. An open-drain power good signal
indicates the output is within 90% to 95% of its nominal
voltage.
Features
2.5V to 5.5V Input Supply Voltage
Current Mode COT Control Loop Design
Fast Transient Response
Internal 34m and 18m Synchronous Rectifier
Highly Accurate V
OUT
Regulation Over
Load/Line Range
Robust Loop Stability with Low-ESR C
OUT
Over-Temperature Protection
Applications
Distributed Power Systems
Enterprise Servers, Ethernet Switches & Routers,
and Global Storage Equipment
Telecom & Industrial Equipment
Ordering Information
RT5735
Package Type
WSC : WL-CSP-20B 1.6x2 (BSC)
Note :
Richtek products are :
Marking Information
1ZW
1Z : Product Code
W : Date Code
RoHS compliant and compatible with the current
requirements of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
Simplified Application Circuit
RT5735
V
IN
………
C1
C2
PVDD
AVDD
FB
L1
LX
C3
C4
C5
V
OUT
EN
VSEL
SCL
PG/PGND
SDA
AGND PGND
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation
DS5735-00
August 2014
www.richtek.com
1
RT5735
Pin Configurations
(TOP VIEW)
A1
VSEL
B1
SDA
C1
AGND
D1
AVDD
E1
PVDD
A2
EN
B2
PGND
C2
PGND
D2
PVDD
E2
PVDD
A3
SCL
B3
A4
FB
B4
PG/PGND PGND
C3
PGND
D3
LX
E3
LX
C4
PGND
D4
LX
E4
LX
WL-CSP-20B 1.6x2 (BSC)
Functional Pin Description
Pin No.
A1
Pin Name
VSEL
Pin Function
Output Voltage/ Mode Selection. The level determines which of two
programmable, configurations to utilize (operating mode / output voltage).
There is an internal pull down resistor on this pin; could be left open if not used.
Enable Control Input. Active high will enable the part. There is an internal
pull-down resistor on this pin.
I
2
C Clock Input.
Output Voltage Pin.
I
2
C Data Input.
Power Good Open-Drain Output. If not used, it has to be connected to ground
plane.
Power Ground.
Analog Ground should be electrically connected to GND close to the device.
Analog Circuit Input Supply Voltage.
Input Supply Voltage, 2.5V to 5.5V.
Switch Node. The source of the internal high-side power MOSFET, and drain of
the internal low-side (synchronous) rectifier MOSFET.
A2
A3
A4
B1
B3
B2, B4, C2,
C3, C4
C1
D1
D2, E1, E2
EN
SCL
FB
SDA
PG/PGND
PGND
AGND
AVDD
PVDD
D3, D4, E3, E4 LX
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation
www.richtek.com
2
DS5735-00
August 2014
RT5735
Function Block Diagram
EN
PG
AVDD
UVLO
Shutdown
Control
OTP
Ton
LX
PVDD
Error Amplifier
VOUT
+
-
+
R
C
C
COMP
VREF
Comparator
+
-
Logic Control
Driver
LX
Current Limit
Detector
Current
Sense
I
2
C Control
AZC
LX
LX
PGND
SDA
SCL
VSEL
AGND
Operation
The RT5735 is a synchronous low voltage step-down
converter that can support the input voltage range from
2.5V to 5.5Vand the output current can be up to 5A.
The RT5735 uses a constant on-time, current mode
architecture. In steady-state operation, the high-side
N-MOSFET is turned on when the current feedback
reaches COMP level which is the amplified difference
between the reference voltage and the feedback
voltage. The on time of high-side N-MOSFET is
determined by on-time generator which is a function of
input and output voltage. After on-time expires,
high-side MOSFET is turned off and low-side MOSFET
is turned on. Until the low-side current sensing signal
reaches the COMP, the high-side MOSFET is turned
on again. In this manner, the converter regulates the
output voltage and keeps the frequency constant.
The switching frequency is 2.4MHz allows for efficiency
and size optimization when selecting the output filter
components.
The RT5735 reduces the external component count by
integrating the boot recharge MOSFET.
The error amplifier EA adjusts COMP voltage by
comparing the output voltage with the internal I
2
C set
reference voltage. When the load increases, it causes
a drop in the output voltage relative to the reference,
then the COMP voltage rises to allow higher inductor
current to match the load current.
PWM Frequency and Adaptive On Time Control
The on-time can be roughly estimated by the equation :
T
on
=
V
OUT
1
where F
SW
is nominal 2.4MHz
V
IN
F
SW
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation
DS5735-00
August 2014
www.richtek.com
3
RT5735
Auto-Zero Current Detector
The auto-zero current detector circuit senses the LX
waveform to adjust the zero current threshold voltage.
When the current of low-side MOSFET decreases to
the zero current threshold, the low-side MOSFET turns
off to prevent negative inductor current. In this way, the
zero current threshold can adjust for different condition
to get better efficiency.
Protection Features
The RT5735 has many features to protect the device.
Under-Voltage Protection (UVLO)
The UVLO continuously monitors the voltage of AVDD
to make sure the device works properly. When the
AVDD is high enough to reach the high threshold
voltage of UVLO, the step-down converter softly starts
or pre-bias to its regulated output voltage. When the
AVDD decreases to its low threshold (180mV
hysteresis), the device will shut down.
Power Good
When the output voltage is higher than PG rising
threshold, the PG flag is High.
Output Under-Voltage Protection (UVP)
When the output voltage is lower than 400mV after
soft-start end is ok, the UVP is triggered. When UVP
occurs, the device enters hiccup mode.
Over-Current Protection (OCP)
The RT5735 senses the current signal when low-side
MOSFET turns on and uses a valley current limiting
circuit. As a result, the OCP set point is the OCP DC
limit minus half of the ripple current. The OCP is
cycle-by-cycle limit. If the OCP occurs, the converter
holds off the next on pulse until inductor current drops
below the OCP limit. If the OCP keeps and the load
current is larger than the current provided by the
converter, the output voltage drops. When the output
voltage triggers UVP, the converter enters hiccup
mode.
Soft-Start
An internal current source charges an internal capacitor
to build the soft-start ramp voltage. The typical
soft-start time is 130s.
Over-Temperature Protection (OTP)
The RT5735 has over-temperature protection. When
the device triggers the OTP, the device shuts down.
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation
www.richtek.com
4
DS5735-00
August 2014
RT5735
Absolute Maximum Ratings
(Note 1)
Supply Input Voltage, V
IN
-------------------------------------------------------------------------------------0.3V to 6V
Other Pins -------------------------------------------------------------------------------------------------------0.3V to V
IN
+ 0.3V
Power Dissipation, P
D
@ T
A
= 25C
WL-CSP-1.6x2-20B ---------------------------------------------------------------------------------------------1.818W
Package Thermal Resistance
(Note 2)
WL-CSP-1.6x2-20B (BSC),
JA
------------------------------------------------------------------------------55C/W
WL-CSP-1.6x2-20B (BSC),
JC
------------------------------------------------------------------------------7C/W
Lead Temperature (Soldering, 10 sec.) --------------------------------------------------------------------260C
Junction Temperature ------------------------------------------------------------------------------------------150C
Storage Temperature Range ---------------------------------------------------------------------------------65C to 150C
ESD Susceptibility
(Note 3)
HBM (Human Body Model) -----------------------------------------------------------------------------------2kV
Recommended Operating Conditions
(Note 4)
Supply Input Voltage -------------------------------------------------------------------------------------------2.5V to 5.5V
Ambient Temperature Range---------------------------------------------------------------------------------40C to 85C
Junction Temperature Range --------------------------------------------------------------------------------40C to 125C
Electrical Characteristics
(V
IN
= 3.6V, T
A
= 25C, unless otherwise specified)
Parameter
Supply Current
Operating Quiescent Current
PWM
Operating Quiescent Current
PFM
Product in Off Mode
DC/DC Converter
Operation Input Voltage
Output Voltage DC Error
Switching Frequency
R
ON
High-Side
Low-Side
Symbol
Test Conditions
Min
Typ
Max
Unit
I
Q_PWM
I
Q_PFM
I
OFF
PWM Mode IQ at I
OUT
= 0A
(Note 5)
PSM Mode IQ at I
OUT
= 0A
EN, VSEL Low, V
IN
= 2.5V to 5.5V
--
--
--
10
70
1
20
--
5
mA
A
A
V
IN
VOUT
F
SW
R
ON_H
R
ON_L
V
IN
= 5V
V
IN
= 5V
I
OC
= 00 Valley Current
(Note 5)
(Note 5)
(Note 5)
PWM Mode
PSM Mode
2.5
--
--
--
2.4
34
18
4
4.7
5.4
6.4
5.5
1
2
2.64
--
--
--
--
--
7.4
V
%
MHz
m
1
1
2.16
--
--
--
--
--
5.4
Valley Current Limit Level
I
LIM,Valley
I
OC
= 01 Valley Current
I
OC
= 10 Valley Current
I
OC
= 11 Valley Current
A
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation
DS5735-00
August 2014
www.richtek.com
5
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