Datasheet

UNISONIC TECHNOLOGIES CO., LTD
UD38251
CMOS IC
38V SYNCHRONOUS BUCK
CONVERTER

DESCRIPTION
The UTC UD38251 is a monolithic synchronous buck regulator.
The device integrates two internal power MOSFETs, and provides
2.5A of continuous load current over a wide input voltage of 8V to
38V. Current mode control provides fast transient response and
cycle-by-cycle current limit.
An internal soft-start prevents inrush current at turn-on, provides
a very compact solution with minimal external components.

HSOP-8
FEATURES
* Wide 8V to 38V Operating Input Range
* Integrated 140mΩ Power MOSFET Switches
* Output Adjustable from VFB(1.00V±2%) to 20V
* Up to 93% Efficiency
* Internal Soft-Start
* Stable with Low ESR Ceramic Output Capacitors
* Fixed 160KHz Frequency
* Cycle-by-Cycle Over Current Protection
* Input Under/Over Voltage Lockout

ORDERING INFORMATION
Ordering Number
UD38251G-SH2-R
Package
HSOP-8
Packing
Tape Reel
UD38251G-SH2-R

(1)Packing Type
(1) R: Tape Reel
(2)Package Type
(2) SH2: HSOP-8
(3)Green Package
(3) G: Halogen Free and Lead Free
MARKING
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UD38251

CMOS IC
PIN CONFIGURATION
BS 1
8 VDD
IN 2
7 EN
GND
SW 3
6 COMP
5
GND 4

PIN DESCRIPTION
PIN NO.
PIN NAME
1
BS
2
IN
3
SW
4
GND
PAD
5
FB
6
COMP
7
8
EN
VDD

FB
DESCRIPTION
Boot-Strap Pin. Supply high side gate driver. Decouple this pin to LX pin with 24ohm +
0.1uF ceramic cap.
Power Input pin. Bypass IN to GND with a suitably large capacitor to eliminate noise on
the input to the IC.
Power Switching Output. SW is the switching node that supplies power to the output.
Connect the output LC filter from SW to the output load.
Ground.
Ground (Connect to GND).
Feedback Input. FB senses the output voltage to regulate that voltage. Drive FB with a
resistive voltage divider from the output voltage.
Compensation Node. COMP is used to compensate the regulation control loop.
Connect a series RC network from COMP to GND to compensate the regulation control
loop.
Enable control. Pull high to turn on. Do not float.
Internal regulator pin
BLOCK DIAGRAM
IN
VDD
Internal
Regulator
CURRENT
SENSE
AMPLIFIER
Input UVLO
7.0V
VDD
BS
EN
M1
2.5V
1.1V
FB
PWM Control
&
Protect Logic
SW
1V
M2
COMP
Softstart
Thermal
Protection
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GND
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UD38251

CMOS IC
ABSOLUTE MAXIMUM RATING (TA=25°C, unless otherwise specified)
PARAMETER
SYMBOL
RATINGS
UNIT
Supply Voltage
VIN
-0.3 ~ +42
V
Switch Node Voltage
VSW
-0.3 ~ VIN+0.3
V
Boost Voltage
VBS
VSW-0.3 ~ VSW+ 6
V
All Other Pins Voltage
-0.3 ~ +6
V
Lead Temperature
TL
260
°C
Junction Temperature
TJ
150
°C
Storage Temperature
TSTG
-65 ~ +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.

RECOMMENDED OPERATING CONDITIONS
PARAMETER
Input Supply Voltage
Output Voltage
Ambient Temperature

SYMBOL
VIN
VOUT
TA
RATINGS
8 ~ 38
VFB ~ 20
-40 ~ +85
UNIT
V
V
°C
THERMAL CHARACTERISTICS
PARAMETER
SYMBOL
RATINGS
Junction To Ambient
θJA
40
Junction to Case
θJC
15
Note: θJA is measured with the PCB copper area of approximately 1 in2(Multi-layer). That need connect to
pad.

UNIT
°C/W
°C/W
exposed
ELECTRICAL CHARACTERISTICS (VIN=12V, TA=25°C, unless otherwise specified.)
PARAMETER
Input Voltage Range
Shutdown Supply Current
Quiescent Current
Feedback Voltage
Feedback Overvoltage Threshold
High-Side Switch On Resistance (Note)
Low-Side Switch On Resistance (Note)
High-Side Switch Leakage Current
Upper Switch Current Limit
Lower Switch Current Limit
Oscillation Frequency
Short Circuit Oscillation Frequency
Maximum Duty Cycle
Minimum On Time (Note)
EN Lockout Threshold Voltage
EN Lockout Hysterisis
Input Under Voltage Lockout Threshold
Input Under Voltage Lockout Threshold
Hysteresis
Input Over Voltage Lockout Threshold
Input Over Voltage Lockout Threshold
Hysteresis
Soft-Start Period
Thermal Shutdown
Thermal Shutdown Hysterisis
Note: Guaranteed by design.
SYMBOL
TEST CONDITIONS
VIN
ISD
VEN= 0V
ICCQ
VEN=5.0V, VFB=1.05V
VFB
8V≤VIN≤38V
OVP_FB
RDS(ON)_1
RDS(ON)_2
VEN=0V, VSW=0V
Minimum Duty Cycle
From Drain to Source
FOSC1
FOSC2
VFB=<0.5V
DMAX
TON_MIN
ENH_LOCK
UVLO
UVLO-Hys
0.98
TYP
0.7
0.7
1.00
1.1X
150
140
MAX
38
1.3
1.5
1.02
10
2.9
6.5
3.5
0.9
160
60
90
220
2.5
210
7.0
7.5
UNIT
V
mA
mA
V
VFB
mΩ
mΩ
μA
A
A
KHz
KHz
%
ns
V
mV
V
800
mV
40
V
OVLO-Hys
5
V
TSD
TSH
3
150
30
ms
°C
°C
OVLO
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VIN Rising
MIN
8
VIN Rising
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UD38251

CMOS IC
FUNCTION DESCRIPTIONS
The UTC UD38251 is a synchronous rectified, current-mode, step-down regulator. It regulates input voltages
from 8V to 38V down to an output voltage as low as VFB, and supplies up to 2.5A of load current.
The UTC UD38251 uses current-mode control to regulate the output voltage. The output voltage is measured at
FB through a resistive voltage divider and amplified through the internal Tran conductance error amplifier. The
voltage at the COMP pin is compared to the switch current measured internally to control the output voltage.
The converter uses internal N-Channel MOSFET switches to step-down the input voltage to the regulated output
voltage. Since the high side MOSFET requires a gate voltage greater than the input voltage, a boost capacitor
connected between SW and BS is needed to drive the high side gate. The boost capacitor is charged from the
internal 5V rail when SW is low.
When the UTC UD38251 FB pin exceeds 10% of the nominal regulation voltage of VFB, the over voltage
comparator is tripped and the COMP pin is discharged to GND, forcing the high-side switch off.

APPLICATION INFORMATION
COMPONENT SELECTION
Setting the Output Voltage
The output voltage is set using a resistive voltage divider from the output voltage to FB pin. The voltage divider
divides the output voltage down to the feedback voltage by the ratio.
Thus the output voltage is:
VOUT  VFB 
R1  R2
R2
For example, VFB =1.00V for a 5.0V output voltage, R2 is 10kΩ, and R1 is 40kΩ.
Inductor Selection
The inductor is required to supply constant current to the output load while being driven by the switched input
voltage. A larger value inductor will result in less ripple current that will result in lower output ripple voltage. However,
the larger value inductor will have a larger physical size, higher series resistance, and/or lower saturation current. A
good rule for determining the inductance to use is to allow the peak-to-peak ripple current in the inductor to be
approximately 30% of the maximum switch current limit.
VIN
Inductor
<28V
33uH
<35V
22uH
The choice of which style inductor to use mainly depends on the price vs. size requirements and any EMI
requirements.
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UD38251

CMOS IC
TYPICAL APPLICATION CIRCUIT
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UD38251

TYPICAL CHARACTERISTICS
Efficiency, EFF (%)
CMOS IC
100
90
80
70
60
50
40
30
20
10
0
Quiescent Current vs. Input Voltage
Quiescent Current, ICCQ (mA)
Efficiency Test (VOUT=5V)
12VIN
24VIN
0
0.5
1.0
1.5
2.0
Output Current, IOUT (A)
2.5
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
Power On
VIN=12V, VOUT=5V, ILOAD=0A
0
10
20
40
30
Input Voltage, VIN (A)
Power On
VIN=12V, VOUT=5V, IOUT=2A
VIN
VIN
SW
SW
VOUT
IOUT
VOUT
IOUT
Steady State
VIN=12V, VOUT=5V, ILOAD=0A
SW
VOUT
ISW
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UD38251
CMOS IC
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.
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