UNISONIC TECHNOLOGIES CO., LTD
UC3873
LOW COST POWER-SAVING
MODE PWM CONTROLLER
FOR FLYBACK CONVERTERS
DESCRIPTION
LINEAR INTEGRATED CIRCUIT
The
UC3873
is a high performance current mode PWM
controller ideally suited for low standby power. Low V
DD
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
UC3873
contains protection with automatic recovery
including OLP (over load protection), programming OTP (over
temperature protection), OVP (over voltage protection), UVLO (V
DD
over voltage clamp and under voltage
lockout).To protect the power MOSFET, Gate-drive output is fixed up to 16V max. The
UC3873
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
UC3873
has such applications as: battery charger, power adaptor, set-top box power supplies, ink jet
printers, open-frame SMPS.
FEATURES
* 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
* 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 V
DD
clamp for higher security
ORDERING INFORMATION
Ordering Number
Lead Free
Halogen Free
UC3873L-AG6-R
UC3873G-AG6-R
Package
SOT-26
Packing
Tape Reel
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UC3873
MARKING INFORMATION
LINEAR INTEGRATED CIRCUIT
PIN CONFIGURATION
PIN DESCRIPTION
PIN NO.
1
2
3
4
5
6
PIN NAME
GND
FB
RT
SENSE
V
DD
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.
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QW-R103-068. D
UC3873
BLOCK DIAGRAM
LINEAR INTEGRATED CIRCUIT
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QW-R103-068. D
UC3873
PARAMETER
LINEAR INTEGRATED CIRCUIT
ABSOLUTE MAXIMUM RATINGS
(T
A
=25°C, V
DD
=15V, unless otherwise specified)
SYMBOL
RATINGS
UNIT
Supply Voltage
V
DD
30
V
Input Voltage to FB Pin
V
FB
-0.3 ~ 7
V
Input Voltage to CS Pin
V
SENSE
-0.3 ~ 7
V
Junction Temperature
T
J
+150
C
Operating Temperature
T
OPR
-40 ~ +125
°C
Storage Temperature
T
STG
-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
SYMBOL
V
DD
SYMBOL
I
STR
I
OP
I
CLAMP
V
THD(ON)
V
DD(MIN)
V
DD(HY)
V
FB-OPEN
A
VCS
V
FB(OUT)
V
FB(END)
V
FB(IN)
F
SW
D
MAX
F
J(SW)
F
DV
F
DT
R
FB
V
OVP
V
OLP
T
Delay
T
SS
V
CS-F
V
CS-V
T
LEB
V
OL
V
OH
t
R
t
F
I
RT
V
TH_OTP
RATINGS
10 ~ 24
MIN
TYP
2.5
1.5
15
12.5
7.5
13.8
8.2
5.6
5
3
1.6
2.4
1.5
65
22
76
MAX
20
2.5
UNIT
V
UNIT
μA
mA
mA
V
V
V
V
V/V
V
V
V
KHz
KHz
%
%
%
%
KΩ
V
V
mS
mS
V
V
ns
0.8
8
500
160
95
0.95
100
1
105
1.05
V
V
ns
ns
uA
V
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Supply Voltage
ELECTRICAL CHARACTERISTICS
(T
A
=25°C, V
DD
=15V，unless otherwise specified)
TEST CONDITIONS
V
DD
= V
DD(ON)
-0.1V
V
FB
=3.5V
V
DD
>V
OVP
PARAMETER
SUPPLY SECTION
Start Up Current
IC Operating current
V
DD
Zener Clamp current
UNDER-VOLTAGE LOCKOUT SECTION
Start Threshold Voltage
Min. Operating Voltage
Hysteresis
CONTROL SECTION
V
FB
Open Loop Voltage Level
PWM Input Gain
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
V
CC
Over Voltage Protection Threshold
FB PIN Over Load Protection Threshold
Over Load Protection Delay-Time
Soft start time
CURRENT LIMITING SECTION
Peak Current Flat Threshold Voltage
Peak Current Valley Threshold Voltage
Lead Edge Blanking Time
DRIVER OUTPUT SECTION
Output Voltage Low State
Output Voltage High State
Output Voltage Rise Time
Output Voltage Fall Time
RT SECTION
Output current of RT pin
Threshold voltage for OTP
www.unisonic.com.tw
14.5
9.5
∆V
FB
/∆V
CS
V
SENSE
=0
V
SENSE
=0
V
SENSE
=0
V
FB
=3.5V
Before enter burst mode
V
FB
=3.5V, V
SENSE
=0
V
DD
=12V~20V
T=-20~100°C
60
70
-4
1.5
25
28
4.4
55
3
0.92
0.80
300
+4
5
5
V
FB
=4.2V
V
FB
=4.2V, Duty≥60%
V
FB
=4.2V, Duty=0%
V
DD
=15V,I
O
=-20mA
V
DD
=12V,I
O
= 20mA
C
L
=1.0nF
C
L
=1.0nF
UNISONIC TECHNOLOGIES CO., LTD
UC3873
OPERATION DESCRIPTION
LINEAR INTEGRATED CIRCUIT
The
UC3873
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 V
DD
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
UC3873
enter burst mode at standby condition to minimize the switching loss and reduces the standby
power consumption. Power supplies using the
UC3873
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 P
OUT
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 f
SW
and P
OUT
/P
OUT (MAX)
as followed Fig.1.
Fig.1 The relation curve between f
SW
and relative output power P
OUT
/ P
OUT (MAX)
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