CURRENT MODE PWM CONTROLLER
DESCRIPTION
SD4870 is a current mode PWM controller IC for high performance,
low standby power offline flyback converter application.
In no load or light load condition, the IC operates in Light Load
Mode to reduce switching loss and improve efficiency.
Large value startup resistor could be used in the startup circuit to
minimize the standby current because of low startup current.
SD4870 offers complete protection functions including cycle-by-
cycle over current protection, over load protection, V
DD
input
voltage over voltage and under voltage protection, etc.
Excellent EMI performance is achieved with frequency shuffling
technique and soft switching control at the totem pole gate driver
output.
DIP-8-300-2.54
SOP-8-225-1.27
SOT-23-6L
FEATURES
* Frequency shuffling to improve EMI performance
* Light Load Mode for minimum standby power
* External programmable switching frequency
* 3uA low startup current
* Internal LEB circuit
* VDD over voltage and under voltage protection
* Gate output maximum voltage clamp
* Over current protection
* Over load protection
* SOT-23-6L/SOP8/DIP8 package
APPLICATIONS
* Battery Chargers
* Adapters
* Set-Top Box Power Supplies
ORDERING INFORMATION
Part No.
SD4870TR
SD4870A
SD4870C
SD4870CTR
Package
SOT-23-6L
DIP-8-300-2.54
SOP-8-225-1.27
SOP-8-225-1.27
4870
SD4870A
SD4870C
SD4870C
Marking
Material
Pb free
Pb free
Pb free
Pb free
Packing Type
Tape & Reel
Tube
Tube
Tape & Reel
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.0
2009.09.22
Page 1 of 8
BLOCK DIAGRAM
ABSOLUTE MAXIMUM RATINGS
Characteristics
VDD Voltage
FB Voltage
SENSE Voltage
RI Voltage
Junction Temperature
Lead Temperature
Storage Temperature
Symbol
V
VDD
V
FB
V
SENSE
V
RI
T
j
T
L
T
stg
Rating
25
-0.3~6
-0.3~6
-0.3~6
-20~150
260
-55~160
Unit
V
V
V
V
°C
°C
°C
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.0
2009.09.22
Page 2 of 8
ELECTRICAL CHARACTERISTICS
(Unless otherwise specified, T
amb
=25°C)
Characteristics
V
DD
Startup Current
Operation Current
UVLO Threshold Voltage
UVLO Hysteresis Voltage
V
DD
Clamp Voltage
Feedback
PWM Gain
FB Open Loop Voltage
FB Short Circuit Current
FB OL Threshold Voltage
OL Debounce Time
FB Input Impedance
Maximum Duty Cycle
Current Sense
LEB Time
SENSE Input Impedance
OC Control Delay
OC Detection Threshold
Switching Frequency
Ocsillation Frequency
Frequency Stability With
V
DD
RI External Resistance
Range
Light Load Mode
Frequency
Frequency Shuffling Range
Gate Driver
Output Low Level
Output High Level
Output Clamp Voltage
Level
Output Rising Time
Output Falling Time
V
OL
V
OH
V
OH_CLAMP
T
R
T
F
V
DD
=16V,C
L
=1nF
V
DD
=16V,C
L
=1nF
V
DD
=16V, I
O
=-20mA
V
DD
=16V, I
O
=20mA
--
10
--
--
--
--
--
13
220
70
0.8
--
--
--
--
V
V
V
ns
ns
f
S
Δf
S_VDD
R
RI_RANGE
f
S_LLM
Δf
S_SHUF
R
I
=100kΩ
RI=100kΩ
V
DD
=12~25V,R
I
=100kΩ
60
--
50
--
-3
65
5
100
22
--
70
--
150
--
3
kHz
%
kΩ
kHz
%
T
LEB
Z
SENSE_IN
T
OC
V
SENSE_OC
R
I
=100kΩ
--
--
--
0.7
300
85
75
0.75
--
--
--
0.8
ns
kΩ
ns
V
A
VCS
VF
B_OPEN
I
FB_SHORT
V
FB_OL
T
D_OL
Z
FB_IN
D
MAX
V
DD
=16V, R
I
=100kΩ
V
FB
=3V,V
SENSE
=0V
4
--
FB short connected to
ground
ΔV
FB
/ΔV
SENSE
--
4.5
0.8
--
2
4.8
1
3.8
35
6
75
--
5
1.2
--
--
--
--
V/V
V
mA
V
ms
kΩ
%
IV
DD_ST
IV
DD
V
UVLO
V
UVLOH
VV
DD_CLP
IVDD =10mA
V
DD
=12 V,R
I
=100kΩ
V
DD
=16V, V
FB
=3V,
R
I
=100kΩ
--
--
13.3
--
--
3
2
14.3
6.3
21.5
20
--
15.3
--
--
μA
mA
V
V
V
Symbol
Test Condition
Min.
Typ.
Max.
Unit
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.0
2009.09.22
Page 3 of 8
PIN CONFIGURATION
PIN DESCRIPTIONS
Pin No.
SD4870
1
2
3
4
5
6
SD4870A
SD4870C
8
7
5
4
2
1
3,6
GND
FB
RI
SENSE
V
DD
GATE
NC
--
I
I/O
I
--
O
--
Ground.
Feedback input pin.
Oscillator frequency setting pin.
A resistor connected between RI and GND.
Switch current sense input pin.
Power supply pin.
Gate driver output pin.
Not connect
Pin Name
I/O
Description
FUNCTION DESCRIPTIONS
SD4870 is a current mode PWM controller used in applications for offline flyback converter. The description of
functions is as follows.
Startup Control
Startup current of SD4870 is very low so that IC could start up quickly. A large value startup resistor can be used
in startup circuit to minimize standby power loss yet provides reliable startup in application.
A 2 MΩ, 1/8 W startup resistor is recommended in normal input range.
Frequency Shuffling Control
Frequency shuffling is used in SD4870 to improve EMI performance.
The oscillation frequency is modulated with a random sourse so that the tone energe is spread out. The spead
spectrum minmizes the conduction band EMI and the system design can be easier.
The entire application system design can become simpler.
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.0
2009.09.22
Page 4 of 8
Light Load Mode
In no load or light load condition, major power loss of total power consumption is from switching loss on the
MOSFET transistor switching loss, the core loss of the transformer loss and the loss on the external snubber
circuit loss become the majority in total power loss. The value of those power loss is proportional to switching
actions within a fixed period of time. So reducing number of switching actions leads to reduction of power loss.
SD4870 enters Light Load Mode in no load or light load condition. The gate drive output switches only when
output DC voltage drops below a present level and the switching frequency reduces. Otherwise the gate drive
remains at off state.
Oscillation Frequency Setting
The oscillation frequency is determined by resistor connected between RI and GND. The relationship between
the value of this resistor and frequency are shown below
6500
f
S
=
(kHz)
, where R
RI
is the value of external resistor and its unit is KΩ.
R
RI
Current Sense and LEB
At switching leading edge time, the current spike due to Snubber diode reverse recovery should be chopped off.
And this is available through internal LEB (Leading Edge Blanking) circuit. So that the external RC filter circuit on
SENSE input is no longer required.
During the blanking period, the PWM comparator and OC comparator are disabled and MOSFET transistor
keeps turn-on state if MOSFET turns on. The minimum on time of MOSFET is LEB time.
Gate Drive
GATE pin is connected to external MOSFET’s gate for switch control. Too weak the gate drive ability results in
more switch loss of MOSFET while too strong gate drive compromises the EMI performance.
A good tradeoff is achieved through the totem pole gate drive design with appropriate output ability and dead
time control.
Protection controls
SD4870 offers complete protection functions including cycle-by-cycle over current protection, over load protection,
V
DD
input voltage over voltage and under voltage protection, etc.
Constant output power limit over universal input voltage range is achived with over current protection threshold
line voltage compensation to over current protection threshold.
V
DD
is supplies by transformer auxiliary winding output. It is clamped when V
DD
is higher than clamp threshold
value. The MOSFET is shut down when V
DD
drops below UVLO threshold voltage and IC enters power on
startup sequence thereafter.
When FB input voltage is higher than over load threshold voltage for more than T
D_OL
, the MOSFET is shut
down and V
DD
voltage drops. IC restarts when V
DD
is lower than UVLO threshold voltage.
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.0
2009.09.22
Page 5 of 8
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