ACT355A
Rev1, 09-Jul-08
ActivePSR
TM
Primary Switching Regulators
FEATURES
•
Multiple Patent-Pending Primary Side
Regulation Technology
The ACT355A ensures safe operation with com-
plete protection against all fault conditions. Built-in
protection circuitry is provided for output short-
circuit, line under-voltage, and over-temperature
conditions.
The ACT355A
ActivePSR
is optimized for cost-
sensitive applications, and utilizes Active-semi’s
proprietary primary-side feedback architecture to
provide accurate constant voltage, constant current
(CV/CC) regulation without the need of an opto-
coupler or reference device. Integrated line and
primary inductance compensation circuitry provides
accurate constant current operation despite wide
variations in line voltage and primary inductance.
Integrated output cord resistance compensation
further enhances output accuracy. These products
achieve excellent regulation and transient re-
sponse, yet requires less than 300mW of standby
power.
The ACT355A is optimized for 2.5W to 3.5W appli-
cations (such as 5V/500mA to 5V/700mA CV/CC
chargers). It is available in a tiny SOT23-5 package.
Figure 1:
Typical Application Circuit
HIGH VOLTAGE DC
•
No Opto-coupler
•
Best Constant Voltage, Constant Current
Over All Accuracy
•
Minimum External Components
•
Integrated Line and Primary Inductance
Compensation
•
Integrated Output Cord Resistance Compen-
sation
•
Line Under-Voltage Protection
•
Short Circuit Protection
•
Over Temperature Protection
•
Flyback Topology in DCM Operation
•
Complies with all Global Energy Efficient
Regulations: 0.3W Standby Power and CEC
Average Efficiency
•
SOT23-5 Package
APPLICATIONS
•
Chargers for Cell Phones, PDAs, MP3,
Portable Media Players, DSCs, and Other
Portable Devices and Appliances
+
N
P
N
S
CV/CC
OUTPUT
•
RCC Adapter Replacements
•
Linear Adapter Replacements
•
Standby and Auxiliary Supplies
GENERAL DESCRIPTION
The ACT355A belongs to the high performance, mul-
tiple patent-pending
ActivePSR
TM
Family of univer-
sal-input AC/DC off-line controllers for battery
charger and adapter applications. It is designed for
the flyback topology working in a discontinuous con-
duction mode (DCM). In addition to being the indus-
try’s most accurate primary side regulator, the
ACT355A meets all of the global energy efficiency
regulations (CEC, European Blue Angel, and US
Energy Star standards) while using very few external
components.
+
VDD
SW
R5
N
A
ACT355A
FB
G
R6
Micro Bridge Technology Co.,Ltd.
ACT355A
Rev1, 09-Jul-08
ORDERING INFORMATION
PART NUMBER
ACT355AUC-T
TEMPERATURE RANGE
-40°C to 85°C
PACKAGE
SOT23-5
PINS
5
PACKING
METHOD
TAPE & REEL
TOP MARK
PSRI
PIN CONFIGURATION
FB
1
5
VDD
PSRI
G
2
NC
3
4
SW
SOT23-5
ACT355AUC-T
PIN DESCRIPTIONS
PIN NUMBER
1
2
3
4
5
PIN NAME
FB
G
NC
SW
VDD
Ground.
No Connection.
PIN DESCRIPTION
Feedback Pin. Connect to a resistor divider network from the auxiliary winding.
Switch Driver. Connect this pin to the emitter of the power NPN transistor or
source of the power MOSFET.
Power Supply.
Micro Bridge Technology Co.,Ltd.
ACT355A
Rev1, 09-Jul-08
ABSOLUTE MAXIMUM RATINGS
PARAMETER
VDD to G
Maximum Continuous VDD Current
FB to G
SW to G
Continuous SW Current
Maximum Power Dissipation (derate 5.3mW/˚C above T
A
= 50˚C)
Junction to Ambient Thermal Resistance (θ
JA
)
Operating Junction Temperature
Storage Temperature
Lead Temperature (Soldering, 10 sec)
VALUE
-0.3 to +23.5
20
-0.3 to +6
-0.3 to +23.5
Internally limited
0.53
190
-40 to 150
-55 to 150
300
UNIT
V
mA
V
V
A
W
˚C/W
˚C
˚C
˚C
: Do not exceed these limits to prevent damage to the device. Exposure to absolute maximum rating conditions for long periods may
affect device reliability.
ELECTRICAL CHARACTERISTICS
(V
DD
= 15V, V
OUT
= 5V, L
P
= 2mH, N
P
= 130, N
S
= 10, N
A
= 32, T
A
= 25°C, unless otherwise specified.)
PARAMETER
VDD Turn-On Voltage
VDD Turn-Off Voltage
VDD OVP Trigger Voltage
Supply Current
Start Up Supply Current
Clamp Switching Frequency
Effective FB Feedback Voltage
Output Cord Resistance Compensation
Primary Current Limit
Maximum Duty Cycle
Switch On-Resistance
SW Rise Time
SW Fall Time
SW Off Leakage Current
Over-Temperature Threshold
SYMBOL
V
DDON
V
DDOFF
V
DDOVP
I
DD
I
DDST
V
FB
I
LIM
D
MAX
R
ON
TEST CONDITIONS
V
DD
Rising
V
DD
Falling
MIN
16.4
7
18.2
TYP
19.4
7.5
20.5
0.8
30
MAX
22.0
8
22.5
1.5
55
58
3.520
UNIT
V
V
V
mA
µA
kHz
V
%
mA
V
DD
= 15V, before turn-on
45
FB in Regulation
Maximum Output Power
I
SW
= 10mA
I
SW
= 50mA
1nF load, 15Ω pull-up
1nF load, 15Ω pull-up
Switch in off-state, V
SW
= 22V
80
3.415
3.467
+3.7
320
86
3.5
30
20
1
160
92
5
%
Ω
ns
ns
10
µA
˚C
Micro Bridge Technology Co.,Ltd.
ACT355A
Rev1, 09-Jul-08
FUNCTIONAL BLOCK DIAGRAM
SW
VDD
REGULATOR
&
UVLO
REFERENCE
FB
3.45V
+
DIFFERENTIAL
PREAMP
-
SIGNAL
FILTER
+
-
ERROR
AMPLIFIER
+
+
-
CURRENT
MODE PWM
COMPARATOR
PFWM
CONTROL
DC CORD
COMP
CURRENT
SHAPING
OSCILLATOR
&
RAMP
G
FUNCTIONAL DESCRIPTION
As shown in the
Functional Block Diagram,
the
ACT355A feedback regulation is done via several
circuit blocks to pre-amplify the FB pin error voltage
relative to an internal reference, filter out the switch-
ing transients, and integrate the resulting useful
differential error voltage for current mode PFWM
(Pulse Frequency and Width Modulation) control.
SW is a driver output that drives the emitter of an
external high voltage NPN transistor or N-channel
MOSFET. This emitter-drive method takes advan-
tage of the high V
CBO
of the transistor, allowing a low
cost transistor such as ‘13003 (V
CBO
= 700V) or
‘13002 (V
CBO
= 600V) to be used for a wide AC input
range.
Constant Voltage (CV) Mode
In constant voltage operation, the IC captures the
auxiliary flyback signal at FB pin through a resistor
divider network R5 and R6 in the simplified applica-
tion circuit. The FB pin is pre-amplified against the
reference voltage, and the secondary side output
voltage error is extracted based on Active-Semi's
proprietary filter architecture. This error signal is
then integrated by the Error Amplifier .
When the secondary output voltage is above regu-
lation, the Error Amplifier output voltage decreases
to reduce the switch current. When the secondary
side is below regulation, the Error Amplifier output
voltage increases to ramp up the switch current to
bring the secondary output back to regulation. The
output regulation voltage is determined by the fol-
lowing relationship:
R 5
⎞⎛
N
S
⎞
⎛
⎟ −
V
F
V
OUTCV
=
3 .45 V
× ⎜
1
+
⎟⎜
⎟
R 6
⎠⎜
N
A
⎠
⎝
⎝
Startup Mode
VDD is the power supply terminal for the IC. During
startup, the IC typically draws 30µA supply current.
The bleed resistor from the rectified high voltage
DC rail supplies current to VDD until it exceeds the
V
DDON
threshold of 19.4V. At this point, the IC en-
ters normal operation when switching begins and
the output voltage begins to rise. The VDD bypass
capacitor must supply the IC and the NPN base
drive until the output voltage builds up enough to
provide power from the auxiliary winding to sustain
the VDD. The V
DDOFF
threshold is 7.5V, and there-
fore, the voltage on the VDD capacitor must not
drop more than 10V while the output is charging up.
(1)
where R5 and R6 are top and bottom feedback re-
sistor, N
S
and N
A
are numbers of transformer sec-
ondary and auxiliary turns, and V
F
is the rectifier
diode forward drop voltage at approximately 0.1A
bias. The ACT355A includes internal feedback loop
compensation to simplify application circuit design.
Micro Bridge Technology Co.,Ltd.
ACT355A
Rev1, 09-Jul-08
Constant Current (CC) Mode
When the secondary output current reaches a level
set by the internal current limiting circuit, the IC
enters current limit condition and causes the secon-
dary output voltage to drop. As the output voltage
decreases, so does the flyback voltage in a propor-
tional manner. An Internal current shaping circuitry
adjusts the switching frequency based on the fly-
back voltage so that the transferred power remains
proportional to the output voltage, resulting in a
constant secondary side output current profile. The
energy transferred to the output during each switch-
ing cycle is ½(L
P
× I
LIM2
) ×
η,
where L
P
is the trans-
former primary inductance, I
LIM
is the primary peak
current, and
η
is the conversion efficiency. From
this formula, the constant output current can be
derived:
I
OUTCC
=
1
2
⎛
η
×
f
SW
L
P
×
I
LIM
⎜
⎜
2
⎝
V
OUTCV
⎞
⎟
⎟
⎠
Loop Compensation
The ACT355A integrates loop compensation circuitry
for simplified application design, optimized transient
response, and minimal external components.
Primary Inductance Compensation
The ACT355A includes built-in primary inductance
compensation to maintain constant current regula-
tion despite variations in transformer manufacturing.
Peak Inductor Current Limit Compensation
The ACT355A includes peak inductor current limit
compensation to achieve constant input power over
wide line and wide load range.
Output Cord Resistance Compensation
The ACT355A provides automatic output cord resis-
tance compensation during constant voltage regula-
tion, monotonically adding an output voltage correc-
tion up to a typical correction of 3.2% at full power.
This feature allows for better output voltage accu-
racy by compensating for the output voltage droop
due to the output cord resistance.
(2)
where f
SW
is the nominal switching frequency and
V
OUTCV
is the nominal secondary output voltage.
The constant current operation typically extends
down to lower than 40% of output voltage regula-
tion.
Light Load Mode
When the secondary side output load current de-
creases to an internally set light load level, the IC's
switching frequency is also reduced to save power.
This enables the application to meet all current
green energy standards. The actual minimum
switching frequency is programmable with a small
dummy load (while still meeting standby power).
Short Circuit Mode
When the secondary side output is short circuited,
the ACT355A enters hiccup mode operation. In this
condition, the auxiliary supply voltage collapses and
the VDD voltage drops below the V
DDOFF
threshold.
This turns off the IC and causes it to restart. This
hiccup behavior continues until the short circuit is
removed.
Output Over Voltage Protection
The ACT355A includes output over-voltage protec-
tion circuitry, which shuts down the IC when the out-
put voltage is 40% above the normal regulation volt-
age or when no feedback signal is detected for 8
consecutive switching cycles. The IC enters hiccup
mode when an output over voltage fault is detected.
Micro Bridge Technology Co.,Ltd.
