ACT337
FUNCTIONAL DESCRIPTION CONT’D
in a constant secondary side output current profile.
The energy transferred to the output during each
switching cycle is ½(L
P
× I
LIM2
) ×
η,
where L
P
is the
transformer 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
⎛
0 . 396 V
×
0 . 9
⎞ ⎛
η
×
f
SW
1
⎟ ×⎜
= ×
L
P
× ⎜
⎜
⎟ ⎜
V
2
R
CS
⎝
⎠ ⎝
OUTCV
2
Rev 2, 14-Nov-12
die temperature. The typical over temperature
threshold is 135°C with 20°C hysteresis. When the
die temperature rises above this threshold the
ACT337 is disabled until the die temperature falls
by 20°C, at which point the ACT337 is re-enabled.
TYPICAL APPLICATION
Design Example
The design example below gives the procedure for
a DCM flyback converter using the ACT337. Refer
to Application Circuit in Figure 6, the design for a
charger application starts with the following
specification:
Input Voltage Range
Output Power, P
O
Output Voltage, V
OUTCV
Full Load Current, I
OUTFL
OCP Current, I
OUTMAX
Transformer Efficiency,
η
xfm
System Efficiency CC,
η
system
System Efficiency CV,
η
90VAC - 265VAC, 50/60Hz
10.5W
5.0V
2.1A
2.5A
0.92
0.76
0.77
⎞
⎟
⎟
⎠
(2)
where f
SW
is the switching frequency and V
OUTCV
is
the nominal secondary output voltage.
The constant current operation typically extends
down to lower than 40% of nominal output voltage
regulation.
Primary Inductance Compensation
The ACT337 integrates a built-in proprietary
(patent-pending) primary inductance compensation
circuit to maintain constant current regulation
despite variations in transformer manufacturing.
The compensated range is ±7%.
Primary Inductor Current Limit Compensation
The ACT337 integrates a primary inductor peak
current limit compensation circuit to achieve
constant input power over line and load ranges.
Protection
The ACT337 incorporates multiple protection
functions including over-voltage, over-current and
over-temperature.
Output Short Circuit Protection
When the secondary side output is short circuited,
the ACT337 enters hiccup mode operation. In this
condition, the VDD voltage drops below the V
DDOFF
threshold and the auxiliary supply voltage
collapses. This turns off the ACT337 and causes it
to restart. This hiccup behavior continues until the
short circuit is removed.
Output Over Voltage Protection
The ACT337 includes output over-voltage
protection circuitry, which shuts down the IC when
the output voltage is 40% above the normal
regulation voltage for 4 consecutive switching
cycles. The ACT337 enters hiccup mode when an
output over voltage fault is detected.
Over Temperature Shutdown
The thermal shutdown circuitry detects the ACT337
Innovative Power
TM
-6-
The operation for the circuit shown in Figure 6 is as
follows: the rectifier bridge D1−D4 and the capacitor
C1/C2 convert the AC line voltage to DC. This
voltage supplies the primary winding of the
transformer T1 and the startup resistor R7/R8. The
primary power current path is formed by the
transformer’s primary winding, the NPN transistor,
the ACT337 internal MOSFET and the current
sense resistor R9. The network consisting of
capacitor C4 and diode D6 provides a VDD supply
voltage for ACT337 from the auxiliary winding of the
transformer. C4 is the decoupling capacitor of the
supply voltage and energy storage component for
startup. The diode D8 and the capacitor C5 rectifies
and filters the output voltage. The resistor divider
consisting of R5 and R6 programs the output
voltage.
The minimum and maximum DC input voltages can
be calculated:
2 P
OUT
(
1
−
t
C
)
2 f
L
η
×
C
IN
V
INDCMIN
=
2
2 V
ACMIN
−
=
2
×
90
2
1
2
×
5(
−
4 ms )
2
×
50
−
≈
90 V
77 %
×
2
×
10
μ
F
(3)
V
INDCMAX
=
2
×
V
ACMAX
=
2
×
265
=
375V
(4)
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