®
RT2805A
5A, 36V, 500kHz Current Mode Asynchronous Step-Down
Converter
General Description
The RT2805A is a current mode asynchronous step-down
converter that achieves excellent load and line regulation.
Over a wide input voltage range from 5.5V to 36V and
supports output current up to 5A. The Current mode
operation provides fast transient response and eases loop
stabilization.
An adjustable soft-start reduces the stress on the input
source at startup. In shutdown mode, the regulator draws
only 25μA of supply current. The RT2805A requires a
minimum number of readily available external
components, providing a compact solution. The RT2805A
provides protection functions inducing input under voltage
lockout, cycle-by-cycle current limit, short circuit
protection and thermal shutdown protection.
The RT2805A is available in the SOP-8 (Exposed Pad)
package.
Features
5A Output Current
Wide Operating Input Range 5.5V to 36V
Adjustable Output Voltage from 1.222V to 26V
High Efficiency up to 90%
Internal Compensation Minimizes External Parts
Count
Internal Soft-Start
110mΩ Internal Power MOSFET Switch
Ω
25μA Shutdown Mode
μ
Fixed 500kHz Frequency
Thermal Shutdown
Cycle-by-Cycle Current Limit
Available In an SOP-8 (Exposed Pad) Package
RoHS Compliant and Halogen Free
Applications
Ordering Information
RT2805A
Package Type
SP : SOP-8 (Exposed Pad-Option 2)
Lead Plating System
G : Green (Halogen Free and Pb Free)
Note :
Richtek products are :
½
Distributive Power Systems
Battery Charger
DSL Modems
Pre-regulator for Linear Regulators
Marking Information
RT2805AGSP : Product Number
RT2805A
GSPYMDNN
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
Simplified Application Circuit
V
IN
VIN
C
IN
BOOT
RT2805A
SW
D1
EN
GND
FB
R2
C
BOOT
L1
R1
C
OUT
V
OUT
Chip Enable
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS2805A-03 August 2015
www.richtek.com
1
RT2805A
Pin Configurations
(TOP VIEW)
BOOT
NC
NC
FB
2
3
4
8
SW
VIN
GND
EN
GND
9
7
6
5
SOP-8 (Exposed Pad)
Functional Pin Description
Pin No.
1
2, 3
4
5
6,
9 (Exposed Pad)
7
8
Pin Name
BOOT
NC
FB
EN
GND
VIN
SW
Pin Function
Bootstrap Input for High Side Gate Driver. Connect a 10nF or greater capacitor
from SW to BOOT to power the high side switch.
No Internal Connection.
Feedback Input. The feedback threshold is 1.222V.
Enable Input. EN is a digital input that turns the regulator on or off. Drive EN
higher than 1.4V to turn on the regulator, lower than 0.4V to turn it off. For
automatic startup, leave EN unconnected.
Ground. The exposed pad must be soldered to a large PCB and connected to
GND for maximum power dissipation.
Power Input. A suitable large capacitor should be connected from the VIN to
GND to eliminate noise on the input to the IC.
Switch Node. Note that a capacitor is required from SW to BOOT to power the
high side switch.
Function Block Diagram
VIN
Ramp
Generator
Current Sense
Amplifier
-
+
BOOT
EN
Regulator
Oscillator
500kHz
+
S
Q
Driver
R
-
Current
Comparator
Bootstrap
Control
GND
Reference
FB
12k
Error
Amplifier
+
-
SW
400k
30pF
13pF
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
2
DS2805A-03 August 2015
RT2805A
Operation
The RT2805A is a constant frequency, current mode
asynchronous 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 N-MOSFET
is turned off, the inductor current conducts through the
external diode.
Error Amplifier
The error amplifier adjusts its output voltage by comparing
the feedback signal (V
FB
) with the internal 1.222V
reference. When the load current increases, it causes a
drop in the feedback voltage relative to the reference, the
error amplifier's output voltage then rises to allow higher
inductor current to match the load current.
Oscillator
The internal oscillator runs at fixed frequency 500kHz. In
short circuit condition, the frequency is reduced to 150kHz
for low power consumption.
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
1.4V and turned off when the EN pin is lower than 0.4V.
When the EN pin is open, it will be pulled up to logic-high
by 1μA current internally.
Soft-Start (SS)
An internal current source charges an internal capacitor
to build a soft-start ramp voltage. The FB voltage will track
the internal ramp voltage during soft-start interval. The
typical soft-start time is 5ms.
Thermal Shutdown
The over temperature protection function will shut down
the switching operation when the junction temperature
exceeds 150°C. Once the junction temperature cools
down by approximately 30°C, the converter will
automatically resume switching.
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS2805A-03 August 2015
www.richtek.com
3
RT2805A
Absolute Maximum Ratings
(Note 1)
−0.3V
to 40V
−0.3V
to (V
IN
+
0.3V)
(V
SW
−
0.3V) to (V
SW
+
6V)
−0.3V
to 6V
2.04W
Supply Voltage, VIN -----------------------------------------------------------------------------------------
Switching Voltage, SW -------------------------------------------------------------------------------------
BOOT Voltage -------------------------------------------------------------------------------------------------
Other Pins ------------------------------------------------------------------------------------------------------
Power Dissipation, P
D
@ T
A
= 25°C
SOP-8 (Exposed Pad) --------------------------------------------------------------------------------------
Package Thermal Resistance (Note 2)
SOP-8 (Exposed Pad),
θ
JA
--------------------------------------------------------------------------------- 49°C/W
SOP-8 (Exposed Pad),
θ
JC
-------------------------------------------------------------------------------- 15°C/W
Junction Temperature ---------------------------------------------------------------------------------------- 150°C
Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------ 260°C
Storage Temperature Range -------------------------------------------------------------------------------
−65°C
to 150°C
ESD Susceptibility (Note 3)
HBM (Human Body Model) --------------------------------------------------------------------------------- 2kV
Recommended Operating Conditions
(Note 4)
Supply Voltage, VIN ----------------------------------------------------------------------------------------- 5.5V to 36V
Junction Temperature Range -------------------------------------------------------------------------------
−
40°C to 125°C
Ambient Temperature Range -------------------------------------------------------------------------------
−
40°C to 85°C
Electrical Characteristics
Parameter
Reference Voltage
High Side Switch-On Resistance
Low Side Switch-On Resistance
Current Limit
Oscillator Frequency
Short Circuit Frequency
Maximum Duty Cycle
Minimum On-Time
Under Voltage Lockout Threshold
Rising
Under Voltage Lockout Threshold
Hysteresis
Logic-High
EN Threshold
Voltage
Logic-Low
Enable Pull Up Current
Shutdown Current
Quiescent Current
Soft-Start Period
Thermal Shutdown
(V
IN
= 12V, T
A
=
−40°C
to 85°C unless otherwise specified)
Symbol
V
REF
R
DS(ON)1
R
DS(ON)2
I
LIM
f
OSC
D
MAX
t
ON
Test Conditions
T
A
= 25°C
I
OUT
= 0A to 5A
Bias Gate Driver at V
IN
= 5.5V
Bias Gate Driver at V
IN
= 5.5V
Voltage Mode Test
V
FB
= 0.8V
V
FB
= 0V
V
FB
= 0.8V
Come from Maximum Duty Cycle
V
IN
Rising, Check Switching
V
IN
Falling, Check Switching
Min
Typ
Max
Unit
V
m
A
kHz
kHz
%
ns
V
mV
V
A
A
mA
ms
C
1.202 1.222 1.239
1.196 1.222 1.245
--
--
6
400
--
85
--
--
--
1.4
--
--
--
--
3
--
110
10
7.5
500
150
90
100
5.2
700
--
--
1
25
0.6
5
160
230
15
9
600
--
95
150
5.5
--
--
0.4
--
--
1
10
--
V
IH
V
IL
I
SHDN
I
Q
T
SD
Let EN = 1.4V, Check I
Q
Let EN = 0.4V, Check I
Q
V
EN
= 0V
V
EN
= 2V, V
FB
= 1.5V
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS2805A-03 August 2015
RT2805A
Note 1.
Stresses beyond those listed
“Absolute
Maximum Ratings” may cause permanent damage to the device. These are
stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in
the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may
affect device reliability.
Note 2.
θ
JA
is measured at T
A
= 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7.
θ
JC
is
measured at the exposed pad of the package. The EVB board copper area is 70mm
2
.
Note 3.
Devices are ESD sensitive. Handling precaution is recommended.
Note 4.
The device is not guaranteed to function outside its operating conditions.
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS2805A-03 August 2015
www.richtek.com
5
