Datasheet

UNISONIC TECHNOLOGIES CO., LTD
UC3656
CMOS IC
1.6MHz, 1A SYNCHRONOUS
BUCK CONVERTER

DESCRIPTION
The UTC UC3656 is a synchronous buck converter for a DC/DC
converter optimized for high-performance microprocessor
applications.
It is configured with current mode for excellent line and load
transient response. At 1.6MHz operation frequency, the use of
small surface mount inductor and capacitor is enabled. The built-in
synchronous switch saves the use of an external Schottky diode
and increases efficiency.
The UTC UC3656 is a low-noise synchronous step-down
DC/DC converter that is ideally suited for systems requiring
supplies from a 1-cell Li-ion battery or from a 3-cell to 4-cell NiCd,
NiMH, or alkaline battery.
The UTC UC3656 is generally applied in power supply for
portable devices and USB-based power system.

FEATURES
* High Efficiency (95%) Possible
* Input Voltage Range: 2.5V ~ 5.5V
* Schottky Diode is Saved
* Output Current: 1000mA (@5V Input 3.3V Output)
* 1.6MHz Oscillation Frequency
* Internal ON/OFF
* 100% Duty Cycle Allows Low Dropout Operation
* Stand-by Current: 1μA (Max.)

ORDERING INFORMATION
Ordering Number
UC3656G-AF5-R

Package
SOT-25
Packing
Tape Reel
MARKING
www.unisonic.com.tw
Copyright © 2015 Unisonic Technologies Co., Ltd
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UC3656
CMOS IC

PIN CONFIGURATION

PIN DESCRIPTION
PIN NO.
1
2
3
4
5

PIN NAME
Run
GND
SW
VIN
FB
DESCRIPTION
ON/OFF Control
Power Ground
Switch Node Connection to Inductor
Power Supply
Output Feedback
BLOCK DIAGRAM
VIN
EN
EN
Logic
UVLO
Reference
+
+
Current
Sense
Amplifier
Slop Comp
Soft Start
-
M1
UG
0.6V
FB
+
-
+
Error
Amplifier
PWM
Comparator
PWM
Logic
SW
Driver
M2
Oscillator
LG
Clock
DCC
-
GND
+
Zero-Current
Comparator
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
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UC3656

CMOS IC
ABSOLUTE MAXIMUM RATING
PARAMETER
Power Supply Voltage
RATINGS
UNIT
-0.3 ~ +6.0
V
RUN, FB
-0.3 ~ VIN
V
Input Pin Voltage
SW
-0.3 ~ VIN +0.3
V
P-Channel Switch Source Current (DC)
1000
mA
N-Channel Switch Sink Current (DC)
1000
mA
Peak SW Sink and Source Current
1.5
A
Junction Temperature
TJ
+150
°C
Operating Temperature
TOPR
-20 ~ +85
°C
Storage Temperature
TSTG
-55 ~ +150
°C
Note: Absolute maximum ratings are those values beyond which the device could be permanently damaged.
Absolute maximum ratings are stress ratings only and functional device operation is not implied.

SYMBOL
VSS
VIN
VIN
THERMAL DATA
PARAMETER
Junction to Ambient

SYMBOL
θJA
RATINGS
165
UNIT
°C/W
ELECTRICAL CHARACTERISTICS (VIN=3.6V, TA=25°C, unless otherwise specified.)
PARAMETER
Input Supply Range
Quiescent Current (Note)
Current In Standby Mode
Feedback Voltage
Feedback Voltage Line-Regulation
Feedback Variation with Temperature
Feedback Current
Peak Inductor Current (Note)
Maximum Output Current (Note)
Oscillator Frequency
SYMBOL
VIN
IS
IST
VFB
VFB-LINE
IFB
IPK
IO
fOSC1
fOSC2
RPFET
RNFET
ILSW
VRUN
IRUN
RDS(ON) of P-Channel MOSFET
RDS(ON) of N-Channel MOSFET
SW Leakage Current
RUN Threshold
RUN Leakage Current
I
( V  VOUT )  t ON
Note: IPK  IO 
 IO  IN
2
2L
TEST CONDITIONS
FB=0.6V, ILOAD=0A, Active Mode
RUN=0V, VIN=5.5V
TA=-20°C ~ +85°C
VCC=2.5V ~ 5.5V
TA=-20°C ~ +85°C
VIN=5V, VOUT=3.3V
VIN=5V±5%, VOUT=3.3V, L=4.7µH
TYP
0.588
300
0.5
0.6
0.1
0.5
1.10
1.25
0.8
1.6
200
0.4
0.35
±0.1
1.0
±0.1
FB=0V
ISW =600mA
ISW =-600mA
RUN=0V, SW=0Vor 5V, VIN=5V
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
MIN
2.5
0.3
MAX UNIT
5.5
V
µA
1
µA
0.612
V
0.5
%
1.0
%
±200
nA
1.50
A
1000 mA
2.0
MHZ
KHZ
0.5
Ω
0.45
Ω
±1
µA
1.5
V
±1
µA
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UC3656

CMOS IC
TYPICAL APPLICATION CIRCUIT
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UC3656

CMOS IC
APPLICATION INFORMATION
Capacitor Selection
Proper selection of capacitors using the UTC UC3656 plays a critical role for system performance. The source
current of the top MOSFET is square wave of duty cycle in a continuous mode.
The input capacitor is generally used to provide a low impedance loop for the edges of pulsed current drawn by
the UTC UC3656. A load step at the output can induce ringing at the input VIN. This ringing can couple to the output
and be mistaken as loop instability. Increasing the capacitance of the input capacitor can great improve the
oscillation. A typical value is 4.7~ 22μF ceramic (X5R or X7R), POSCAP or Aluminum Polymer. With these
capacitors, better high frequency bypassing is achieved; resistive losses are thus reduced by their low ESR for
higher efficiency. Moreover, the input capacitor RMS current changes with the input voltage and the output voltage.
The maximum RMS current in the input capacitor can be expressed in the following formula:
IRMS  IOMAX
VO
V
(1  O )
VIN
VIN
The output capacitor depends upon the suitable ripple voltage. Low ripple voltage corresponds to lower effective
series resistance (ESR).
1
The output ripple voltage can be calculated by: VOUT  IL (ESR 
)
8fCOUT
The formula as follows shows the output capacitor RMS ripple current: IRMS 
VOUT  ( VIN  VOUT )
L  f  VIN
2 3
1
For the UTC UC3656, X5R or X7R dielectric formulations have the priority in choosing the input and output
ceramic capacitors. This is because these dielectrics provide the best temperature and voltage characteristics of all
the ceramics for a given value and size.
Inductor Selection
Selecting an optimum inductor is dependent upon the expected ripple current and large value inductors lower
ripple current and small value inductors result in higher ripple current. In addition, the losses associated with the
DCR and its effect on the total converter efficiency should be taken consideration.
The inductor is also chosen to limit the ripple current to some pre-fixed value, approx. 20 ~ 40% (TYP.) of the
full load current at the maximum input voltage.
The inductance value can be calculated in the following equation:
IL  0.2 ~ 0.4  IOUT(MAX)
L
VOUT
V
(1  OUT )
f  IL
VIN
Adjustable Output Voltage
A resistive divider adjusts the output voltage of the UTC UC3656, which divides a 0.6V reference voltage at FB
pin. The output voltage can be obtained by the equation as follows:
R2
VOUT  0.6V  (1 
)
R1
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
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UC3656

CMOS IC
TYPICAL CHARACTERISTICS
VIN=3.6V, VRUN=2.V, L=4.7uH, R2=300KΩ, R1=150KΩ)
CH1: SW, CH2: VO, CH3: IO=200mA
CH1: SW, CH2: VO, CH3: IO=500mA
CH1
0
CH1: 2V/div
CH2: 50mV/div
CH3: 0.5A/div
CH1: 2V/div
CH2: 50mV/div
CH3: 0.5A/div
CH1
0
CH2
0
0
CH2
CH3
0
0
CH3
4000
0
2000
Time (400ns/div)
4000
Efficiency, EFF (%)
2000
Time (400ns/div)
CH1: 2V/div
CH2: 50mV/div
CH3: 0.5A/div
0
UTC assumes no responsibility for equipment failures that result from using products at values that
exceed, even momentarily, rated values (such as maximum ratings, operating condition ranges, or
other parameters) listed in products specifications of any and all UTC products described or contained
herein. UTC products are not designed for use in life support appliances, devices or systems where
malfunction of these products can be reasonably expected to result in personal injury. Reproduction in
whole or in part is prohibited without the prior written consent of the copyright owner. The information
presented in this document does not form part of any quotation or contract, is believed to be accurate
and reliable and may be changed without notice.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
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