Datasheet

UNISONIC TECHNOLOGIES CO., LTD
UC3873B
LINEAR INTEGRATED CIRCUIT
LOW COST POWER-SAVING
MODE PWM CONTROLLER
FOR FLYBACK CONVERTERS

6
DESCRIPTION
5
4
3
2
The UC3873B is a high performance current mode PWM
1
controller ideally suited for low standby power. Low VDD startup
SOT-26
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 UC3873B contains protection with automatic recovery
including OLP (over load protection), programming OTP (over
temperature protection), OVP (over voltage protection), UVLO (VDD over voltage clamp and under voltage
lockout).To protect the power MOSFET, Gate-drive output is fixed up to 16V max. The UC3873B contains protection
OCP (cycle-by-cycle current limiting).
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 UC3873B has such applications as: battery charger, power adaptor, set-top box power supplies, ink jet
printers, open-frame SMPS.

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
* OLP,OVP and VDD clamp for higher security
* -500/+500mA gate driver

* Programming OTP for higher security
* Fixed switch frequency 65kHz
* Gate output voltage clamped at 15V
* Low start-up current
* Cycle-by-cycle Current Limiting
* Under voltage lockout (UVLO)
* Few external components required
ORDERING INFORMATION
Ordering Number
UC3873BG-AG6-R
Package
SOT-26
Packing
Tape Reel
UC3873BG-AG6-R
(1)Packing Type
(1) R: Tape Reel
(2)Package Type
(2) AG6: SOT-26
(3)Green Package
(3) G: Halogen Free and Lead Freee
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UC3873B
LINEAR INTEGRATED CIRCUIT

MARKING

PIN CONFIGURATION

PIN DESCRIPTION

PIN NO.
1
PIN NAME
GND
2
FB
3
RT
4
SENSE
5
6
VDD
GATE
PIN TYPE
DESCRIPTION
P
Ground.
Feedback input pin. The PWM duty cycle is determined by voltage level
I
into this pin and SENSE pin input.
I
Connected through a NTC resistor to GND for OTP.
Current sense input pin. Connected to MOSFET current sensing resistor
I
node.
P
Power supply.
O
The totem-pole output driver for driving the power MOSFET.
BLOCK DIAGRAM
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UC3873B

LINEAR INTEGRATED CIRCUIT
ABSOLUTE MAXIMUM RATINGS (TA=25°C, VDD =15V, unless otherwise specified)
PARAMETER
SYMBOL
RATINGS
UNIT
Supply Voltage
VDD
30
V
Input Voltage to FB Pin
VFB
-0.3 ~ 7
V
Input Voltage to CS Pin
VSENSE
-0.3 ~ 7
V
Junction Temperature
TJ
+150
C
Operating Temperature
TOPR
-40 ~ +125
°C
Storage Temperature
TSTG
-50 ~ +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.

OPERATING RANGE
PARAMETER
Supply Voltage

SYMBOL
VDD
RATINGS
10 ~ 24
UNIT
V
ELECTRICAL CHARACTERISTICS (TA=25°C, VDD=15V,unless otherwise specified)
PARAMETER
SYMBOL
SUPPLY SECTION
Start Up Current
ISTR
IC Operating current
IOP
VCC Zener Clamp Voltage
VCC(clamp)
UNDER-VOLTAGE LOCKOUT SECTION
Start Threshold Voltage
VTHD(ON)
Min. Operating Voltage
VDD(MIN)
CONTROL SECTION
VFB Open Loop Voltage Level
VFB-OPEN
PWM Input Gain
AVCS
Burst-Mode Out FB Voltage
VFB(OUT)
Reduce-Frequency end FB Voltage
VFB(END)
Burst-Mode Enter FB Voltage
VFB(IN)
Normal
Switch Frequency
FSW
Power-Saving
Duty Cycle
DMAX
Frequency Hopping
FJ(SW)
Frequency VDD Stability
FDV
Frequency Temperature Stability
FDT
PROTECTION SECTION
VCC Over Voltage Protection Threshold
VOVP
FB PIN Over Load Protection Threshold
VOLP
Over Load Protection Delay-Time
TDelay
Soft start time
TSS
CURRENT LIMITING SECTION
Peak Current Flat Threshold Voltage
VCS-F
Peak Current Valley Threshold Voltage
VCS-V
Lead Edge Blanking Time
TLEB
DRIVER OUTPUT SECTION
Output Voltage Low State
VOL
Output Voltage High State
VOH
Output Voltage Rise Time
tR
Output Voltage Fall Time
tF
RT SECTION
Output current of RT pin
IRT
Threshold voltage for OTP
VTH_OTP
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TEST CONDITIONS
VDD = VDD(ON)-0.5V
VFB=3.5V
IVCC=20mA
∆VFB/∆VCS
VSENSE =0
VSENSE =0
VSENSE =0
VFB=3.5V
Before enter burst mode
VFB=3.5V, VSENSE=0
MIN
TYP
MAX
UNIT
31
2.5
0.8
33
20
1.8
35
μA
mA
V
12
6
13
7
14
8
V
V
70
V
V/V
V
V
V
KHz
KHz
%
%
%
%
60
5.4
3
1.8
2.9
1.5
65
22
78
-3
VDD=12V~20V
T=-20~100°C
VFB=4.0V
1.5
25
80
VFB=4.0V, Duty≥60%
VFB=4.0V, Duty=0%
VDD=15V,IO=-20mA
VDD=12V,IO= 20mA
CL=1.0nF
CL=1.0nF
0.85
0.55
+3
5
5
27
4.2
88
2.5
29
0.95
0.65
350
1.05
0.75
V
V
ns
0.8
V
V
ns
ns
110
1.10
uA
V
96
11
170
60
100
1.00
105
1.05
V
V
mS
mS
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LINEAR INTEGRATED CIRCUIT
OPERATION DESCRIPTION
The UC3873B devices integrate many useful designs into one controller for low-power switch-mode power
supplies. The following descriptions highlight some of the features of the UC3873 series.
Start-up Current
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 UC3873B enter burst mode at standby condition to minimize the switching loss and reduces the
standby power consumption. Power supplies using the UC3873B can easily meet even the strictest regulations
regarding standby power consumption.
Switch Frequency Set
The maximum switch frequency is fixed to 65KHz. 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.
Fig.1 The relation curve between fSW and relative output power POUT/ POUT (MAX)
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LINEAR INTEGRATED CIRCUIT
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 gate driver.
Constant Output Power Limit
When the SENSE voltage, across the sense resistor RS, reaches the threshold voltage, around 0.8V, the output
GATE drive will be turned off after a small propagation delay tD. This propagation delay will introduce an additional
current proportional to tD×VIN/Lp. Since the propagation delay is nearly constant regardless of the input line voltage
VIN. Higher input line voltage will result in a larger additional current and hence the output power limit is also higher
than that under low input line voltage. To compensate this variation for wide AC input range, the threshold voltage is
adjusted by the VIN current. Since VIN pin is connected to the rectified input line voltage through a resistor RVIN, a
higher line voltage will generate higher VIN current into the VIN pin. The threshold voltage is decreased if the VIN
current is increased. Smaller threshold voltage, forces the output GATE drive to terminate earlier, thus reduce the
total PWM turn-on time and make the output power equal to that of low line input. This proprietary internal
compensation ensures a constant output power limit for wide AC input voltage from 90VAC to 264VAC.
Under Voltage Lockout (UVLO)
The turn-on and turn-off thresholds of the UC3873B are fixed internally at VTHD(ON)/VDD(MIN) During start-up, the
hold-up capacitor must be charged to VTHD(ON) through the start-up resistor, so that the UC3873B 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.
Gate Output
The UC3873B output stage is a fast totem pole gate driver. Cross conduction has been avoided to minimize heat
dissipation, increase efficiency, and enhance reliability. A good tradeoff is achieved through dead time control. The
low idle loss and good EMI system design is easier to achieve with this dedicated control scheme. An internal 15V
clamp is added for MOSFET gate protection at higher than expected VDD input.
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UC3873B
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LINEAR INTEGRATED CIRCUIT
OPERATION DESCRIPTION (Cont.)
Protection Controls
The IC takes on more protection functions such as OVP, OLP and OTP etc. In case of those failure modes for
continual blanking time, the driver is shut down. Driver 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).
PCB Layout Note
Noise from the current sense or the control signal can cause significant pulse width jitter in
continuous-conduction mode, and slope compensation helps alleviate these problems. Good placement and layout
practices should be followed. Avoiding long PCB traces and component leads, locating compensation and filter
components near the UC3873B, and increasing the power MOS gate resistance is advised.
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UC3873B

LINEAR INTEGRATED CIRCUIT
REFRENCE CIRCUIT 12W (5V/2.4A)
BOM
Reference
BD1
C2
C1
C3
C4
C5, C13
C8
C9
C10, C11
C12
D2
D3
D4
D5
F1
L1
L2
Q1
Component
BD 1A/600V
EC 22μF/400V 105°C
EC 4.7μF/400V 105°C
CC 0.01μF/1000V
EC 10μF/50V
CC 104P/25V 0805
CC 102P/25V 0805
CC 222P/100V 1206
EC 470u/16V 105°C
CC 222P/16V 0805
Diode FR107
Diode FR102
Diode 1N4148_SOD-123
Diode SB2040_TO-220F
Fuse 2A/250VAC
10μH 6mm
1mH 6×8mm
4N60_TO-220F
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Reference
R1, R2
R3
R4
R5
R6
R7, R7A
R8
R9
R10
R12
R13
R14
R15, R16
R17
R18
T1
U1
U2
U3
Component
R 1.5MΩ 1206
R 200KΩ 1206
R 0Ω 0805
R 10Ω 0805
R 43Ω 1206
R 2Ω 1206
R 51KΩ 1206
NTC R 12KΩ 0805
R 22Ω 1206
R 820Ω 0805
R 2.2KΩ 0805
R 1KΩ 0805
R 22KΩ 0805 ±1%
R 330KΩ 0805 ±1%
R 220Ω 1206
EE-22
IC UC3873
TL431
PC817B
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UC3873B
LINEAR INTEGRATED CIRCUIT
TYPICAL CHARACTERISTICS
DMAX (%)
FSW-Power-Saving (KHz)
ISTR (µA)
FSW(kHz)
VDD(min) (V)
VTHD(ON) (V)

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UC3873B
LINEAR INTEGRATED CIRCUIT
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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