Datasheet

UNISONIC TECHNOLOGIES CO., LTD
US3706
Preliminary
CMOS IC
CURRENT MODE PWM POWER
SWITCH

DESCRIPTION
The UTC US3706 combines a dedicated current mode PWM
controller with a high voltage power MOSFET. Low VDD startup
current make the power reliable on startup design and a large
value resistor could be used in the startup circuit to minimize the
standby power. At no load condition, the IC operates in
power-saving mode for lower standby power, decreasing frequency
for Higher conversion efficiency at light load condition.
The UTC US3706 offers complete protection coverage with
automatic self-recovery feature including Cycle-by-Cycle current
limiting (OCP), over load protection (OLP), over temperature
protection (OTP), over voltage protection and VDD under
voltage lockout (UVLO).
The internal slope compensation improves system stability at
high PWM duty cycle output. Leading-edge blanking on current
sense input removes the signal glitch, which offering minimal
external component count in the design. Excellent EMI
performance is achieved with UTC proprietary frequency hopping
technique (ZL201020615247.1) together with soft driver control.
Audio noise is eliminated due to switch frequency more than 20kHz
during operation.
The US3706 has such applications as: battery charger, power
adaptor, set-top box power supplies, ink jet printers, open-frame
SMPS.

TO-220F-6
FEATURES
* UTC proprietary frequency hopping technology for
Improved EMI performance.
* Power-saving mode for high light-load and standby
efficiency
* Soft Start
* Dynamic peak current limiting for constant output power
* Built-in synchronized slope compensation

1
* OLP,OVPand OTP for higher security
* Fixed switch frequency 65kHz
* Low start-up current
* Cycle-by-cycle Current Limiting
* Under voltage lockout (UVLO)
* Leading edge blanking on source input
* Few external components required
ORDERING INFORMATION
Ordering Number
Lead Free
US3706L-TF6-T
Halogen Free
US3706G-TF6-T
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Copyright © 2015 Unisonic Technologies Co., Ltd
Package
Packing
TO-220F-6
Tube
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US3706
Preliminary

MARKING

PIN CONFIGURATION

PIN DESCRIPTION
PIN NO.
PIN NAME
1
DRAIN
2
3
4
VDD
Source
RT
5
FB
6
GND

CMOS IC
DESCRIPTION
HV MOSFET Drain Pin. The Drain pin is connected to the primary lead of the
transformer.
Supply voltage
HV MOSFET Source Pin.
Connected through a NTC resistor to ground for over temperature protection.
Feedback input pin. The PWM duty cycle is determined by voltage level into this pin and
the current-sense signal at Pin 3.
Ground
BLOCK DIAGRAM
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US3706

Preliminary
CMOS IC
ABSOLUTE MAXIMUM RATING
PARAMETER
SYMBOL
RATINGS
UNIT
VDD Voltage
-0.3~30
V
FB Input Voltage
-0.3~7
V
Source Input Voltage
-0.3~7
V
Operating Junction Temperature
TJ
150
°C
Storage Temperature
TSTG
-55~150
°C
Lead Temperature (Soldering, 10secs)
TL
260
°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.

ELECTRICAL CHARACTERISTICS (TA=25°C , VDD=16V, RT=100KΩ, unless otherwise noted)
PARAMETER
SUPPLY SECTION
Start Up Current
IC Operating current
VDD Zener Clamp current
UNDER-VOLTAGE LOCKOUT SECTION
Start Threshold Voltage
Min. Operating Voltage
CONTROL SECTION
VFB Open Loop Voltage Level
Burst-Mode Out FB Voltage
Reduce-Frequency end FB Voltage
Burst-Mode Enter FB Voltage
Normal
Switch Frequency
Power-Saving
Duty Cycle
Frequency Hopping
Frequency VDD Stability
Frequency Temperature Stability
Feedback Resistor
PROTECTION SECTION
VCC Over Voltage Protection Threshold
FB PIN Over Load Protection Threshold
Power Limiting Debounce Time
Soft start time
CURRENT LIMITING SECTION
Peak Current Flat Threshold Voltage
Peak Current Valley Threshold Voltage
Lead Edge Blanking Time
RT SECTION
Output current of RT pin
Threshold voltage for OTP
Mosfet Section
MOSFET Drain-Source Breakdown
Voltage
Static Drain to Source On Resistance
SYMBOL
MIN
TYP
MAX
UNIT
26
2.5
2.4
27.5
20
3.0
29
μA
mA
V
VDD(ON)
VDD(MIN)
12
7
13.5
8
15
9
V
V
VFB-OPEN
VFB(OUT)
VFB(END)
VFB(IN)
4.9
5.1
1.6
2.2
1.5
60
22
77
5.3
V
V
V
V
KHz
KHz
%
%
%
%
KΩ
V
V
mS
mS
ISTR
IOP
vCLAMP
FSW
DMAX
FJ(SW)
FDV
FDT
RFB
VDD = VDD(ON)-0.1V
VFB=3.5V
IVDD=10mA
VSENSE =0
VSENSE =0
VSENSE =0
VFB=3.5V
Before enter burst mode
VFB=3.5V, VSENSE=0
VFB=3.5V
VCS-F
VCS-V
TLEB
VFB=4.2V, Duty≥60%
VFB=4.2V, Duty=0%
IRT
55
17
70
-4
17
20
65
27
85
+4
5
10
23
22.5
4.6
100
1
23
4.8
125
2
24.5
5.0
150
3
VDD=12V~20V
T=-20~100°C
VOVP
VOLP
TD_PL
TSS
0.8
0.7
350
VTH_OTP
90
0.9
BVdss
600
Rdson
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
TEST CONDITIONS
100
1
V
V
ns
110
1.1
uA
V
V
1.5
Ω
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US3706

Preliminary
CMOS IC
OPERATION DESCRIPTION
The UTC US3706 is a low power off-line SMPS Switcher optimized for off-line flyback converter. It integrates
many useful designs into one controller for low-power switch-mode power supplies. The following descriptions
highlight some of the features of the US3706 series.
Startup Current and Start up Control
The start-up current is only 2.5μA. Low start-up current allows a start-up resistor with a high resistance and a
low-wattage to supply the start-up power for the controller. For AC/DC adaptor with universal input range design, a
2.5~3MΩ, 1/8W startup resistor could be used together with a VDD capacitor to provide a fast startup and low power
dissipation solution.
Power-Saving Mode Operation
The proprietary Power-Saving Mode function provides linearly decreasing the switching frequency under
light-load conditions for higher efficiency. The feedback voltage, which is sampled from the voltage feedback loop, is
taken as the reference. Once the feedback voltage dropped below the threshold voltage, the switching frequency
starts to decrease. This Power-Saving Mode function dramatically reduces power consumption under light-load
conditions. The 22KHz minimum frequency control also eliminates the audio noise at any loading conditions.
At zero load condition, the magnitude of power loss is in proportion to the number of switching events within a
fixed period of time. Reducing switching events leads to the reduction on the power loss and thus conserves the
energy. The US3706 enter burst mode at standby condition to minimize the switching loss and reduces the standby
power consumption. Power supplies using the US3706 can easily meet even the strictest regulations regarding
standby power consumption.
Switch Frequency Set
The maximum switch frequency is fixed to 60KHz. Switch frequency is modulated by output power POUT during IC
operating. At no load or light load condition, most of the power dissipation in a switching mode power supply is from
switching loss on the MOSFET transistor, the core loss of the transformer and the loss on the snubber circuit. The
magnitude of power loss is in proportion to the number of switching events within a fixed period of time. So lower
switch frequency at lower load, which more and more improve IC’s efficiency at light load. At from no load to light
load condition, The IC will operate at from Burst mode to Reducing Frequency Mode. The relation curve between fSW
and POUT/POUT (MAX) as followed Fig.1.
70
1%~100% Load @ 115 & 230 Vac
60
55
115V
40
230V
25
10
1%
9%
19%
30%
100%
POUT/POUT(max)
Fig.1 The relation curve between fSW and relative output power POUT/ POUT (MAX)
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US3706

Preliminary
CMOS IC
OPERATION DESCRIPTION (Cont.)
Frequency Hopping For EMI Improvement
The Frequency hopping is implemented in the IC; there are two oscillators built-in the IC. The first oscillator is to
set the normal switching frequency; the switching frequency is modulated with a period signal generated by the 2nd
oscillator. The relation between the first oscillator and the 2nd oscillator as followed Fig.2. So the tone energy is
evenly spread out, the spread spectrum minimizes the conduction band EMI and therefore eases the system design
in meeting stringent EMI requirement.
Fig.2 Frequency Hopping
Built-in Slope Compensation
Built-in slope compensation circuit greatly improves the close loop stability at CCM and prevents the
sub-harmonic oscillation.
Leading-Edge Blanking
Each time the power MOSFET is switched on, a turn-on spike will inevitably occur at the sense-resistor. To avoid
premature termination of the switching pulse, a 400ns leading-edge blanking time is built in. Conventional RC
filtering can therefore be omitted. During this blanking period, the current-limit comparator is disabled and it cannot
switch off the power MOSFET.
Under Voltage Lockout (UVLO)
The turn-on and turn-off thresholds of the US3706 are fixed internally at VDD(ON)/VDD(MIN) During start-up, the
hold-up capacitor must be charged to VDD(ON) through the start-up resistor, so that the US3706 will be enabled. The
hold-up capacitor will continue to supply VDD until power can be delivered from the auxiliary winding of the main
transformer. VDD must not drop below VDD(MIN) during this start-up process. This UVLO hysteresis window ensures
that hold-up capacitor will be adequate to supply VDD during start-up.
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5 of 8
QW-R129-001.b
US3706

Preliminary
CMOS IC
OPERATION DESCRIPTION (Cont.)
Protection Controls
The IC takes on more protection functions such as OVP, OLP and OTP etc. In case of those failure occurs for
continual blanking time, the power MOSFET is shut down. Power MOSFET is reset after failure is eliminated.
OVP
The OVP will shut down the switching of the power MOSFET whenever VDD >VOVP. The OVP event as followed
Fig.3.
Fig.3 OVP case
Fig.4 OLP case
OLP
OLP will shut down driver when VFB> VOLP for continual a blanking time. The OLP event as followed Fig.4.
OTP
OTP will shut down driver when the NTC resistor temperature TJ>T (THR).
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QW-R129-001.b
US3706
Preliminary
CMOS IC
TYPICAL APPLICATION CIRCUIT

L
N
LF1
F1 3
1
4
2
R1
R2
LF2
3
3
4
1
4
2
R40 C40
T1
4
BD1
+ 1
R6 C1
2
C2
+
R7
L1
VOUT+
R8
2
D5
C41
+
C43
C42
+
B
R23
VOUT GND
5
8
D1
R13
C7
9
D3
4
C10
U1
U3
6 GND DRAIN 1
5
VDD 2
FB
4 RT
SOURCE 3
C6
R41
RM-8
+
3
1
R45
R42
R43
2
C45 C46
C12
U4
R12
C5
R46
RT1
BOM
Reference
BD1
C2
C3
C6
C10
C7, C46
C11
C43
C45
CX1
CY1
R21
R20
Component
2A_600V
Capacitor, aluminum
electrolytic,33uF/400V, 105℃,±20%
Capacitor, ceramic, 1000pF/1KV, NPO,
SMD1206
Capacitor, ceramic, 1nF/50V, X7R,
SMD0805
Capacitor, aluminum
electrolytic,10uF/50V; 105℃,±20%
Capacitor, ceramic, 0.1uF/50V, X7R,
SMD0805
Capacitor, ceramic, 68pF/50V, X7R,
SMD0805
Capacitor, aluminum electrolytic,
680uF/25V, 105℃,±20%
Capacitor, ceramic, 10NF/50V, X7R,
SMD0805
Capacitor, X2, 0.33uF/275VAC,
105℃ ,±20%
Capacitor, Y1, 1000pF/400V, 105℃,±20%
D5
Capacitor, ceramic, 100pF/200V, ±20%,
SMD1206
Diode , fast recovery, 1N4007,
1.0A/1000V
Diode , fast recovery, BAV20WG,
SOT-123
Diode, MGBR20L60CG, 20A/60V,TO-220
F1
Fuse, 3.15A / 250V
C40
D1
D3
LF1, LF2
choke
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www.unisonic.com.tw
R47
CY1
R22
Reference
R6, R2
Component
Resistor, chip,1.5M, 1/4W,±5%, SMD1206
R7, R8
Resistor, chip,180K, 1/4W,±5%, SMD1206
R13
Resistor, chip,4.7-, 1/8W,±5%, SMD0805
R18
Resistor, chip,1K, 1/8W,±5%, SMD0805
R20, R21
Resistor, chip, 1.8-, 1/4W,±5%, SMD1206
R40
Resistor, chip, 47-, 1/4W,±5%, SMD1206
R41
Resistor, chip,820-, 1/10W,±5%, SMD0603
R42
Resistor, chip,2.2K, 1/10W,±5%, SMD0603
R43
Resistor, chip,300-, 1/10W,±1%, SMD0603
R45
Resistor, chip,39K, 1/10W,±1%, SMD0603
R47
Resistor, chip,10K, 1/10W,±1%, SMD0603
T1
Transformer, RM-8
U1
IC, PWM controller, UTC US3706,
TO-220F-6
U4
IC, TL431 SOT-23
U3
No
Component
PCB
IC, Opto-Coupler, LTV-357-T-C, SMD
C1, C41, C42, C5, R1, R2, R3, R5, R12,
R22, R47
32mmX71mmX1.6mm
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US3706
Preliminary
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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8 of 8
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