
NCP1560
http://onsemi.com
12
Typical Characteristics
Figure 27. Outputs Rise Time versus Load
Capacitance
C
L
, LOAD CAPACITANCE (pF)
200
150
100
50
0
0
10
20
30
40
50
60
80
Figure 28. Outputs Fall Time versus Load
Capacitance
C
L
, LOAD CAPACITANCE (pF)
200
150
100
50
0
0
5
10
15
20
25
35
70
t
o
,
30
t
o
,
T
J
= -40
°
C
T
J
= 25
°
C
T
J
= 125
°
C
T
J
= -40
°
C
T
J
= 25
°
C
T
J
= 125
°
C
175
125
75
25
175
125
75
25
Measured from 10% to 90% of V
OH
V
AUX
= 12 V
Measured from 90% to 10% of V
OH
V
AUX
= 12 V
DETAILED OPERATING DESCRIPTION
The NCP1560 PWM controller contains all the features
and flexibility needed for implementation of Voltage-Mode
Control in high performance DC/DC converters. This
device cost effectively reduces system part count with the
inclusion of a high voltage start-up regulator. The NCP1560
provides two control outputs. Output 1 controls the main
switch of a forward or flyback topology. Output 2 has an
adjustable overlap delay, which can be used to control an
active clamp/reset switch, a synchronous rectifier switch, or
both. Other distinctive features include: two mode
overcurrent protection, line under/over voltage lockout, fast
line feedforward, soft start and a maximum duty cycle limit.
The Functional Block Diagram is shown in Figure 2.
The features included in the NCP1560 provide all the
advantages of Current-Mode Control, fast line feedforward,
and cycle by cycle current limit. It eliminates the
disadvantages of low power jitter, slope compensation and
noise susceptibility.
High Voltage Start-up Regulator
The NCP1560 contains an internal high voltage start-up
regulator that eliminates the need for external start-up
components. In addition, this regulator increases the
efficiency of the supply as it uses no power when in the
normal mode of operation, but instead uses power supplied
by an auxiliary winding.
The start-up regulator consists of a constant current source
that supplies current from the input line voltage (V
in
) to the
capacitor on the V
AUX
pin (C
AUX
). The start-up current is
typically 13.8 mA. Once V
AUX
reaches 11 V, the start-up
regulator turns OFF and the outputs are enabled. When V
AUX
reaches 7 V, the outputs are disabled and the start-up
regulator turns ON. This “7-11” mode of operation is known
as Dynamic Self Supply (DSS). The V
AUX
pin can be biased
externally above 7 V once the outputs are enabled to prevent
the start-up regulator from turning ON. It is recommended
to bias the V
AUX
pin using an auxiliary supply generated out
of an auxiliary winding from the power transformer. An
independent voltage supply can also be used. However, if
V
AUX
is biased before the outputs are enabled or while a
fault is present, the One Shot Pulse Generator (Figure 2) will
not be enabled and the outputs will remain OFF.
As the DSS sources current to the V
AUX
pin, a diode should
be placed between C
AUX
and the auxiliary supply as shown
in Figure 29. This will allow the NCP1560 to charge C
AUX
while preventing the start-up regulator from sourcing current
into the auxiliary supply.
Figure 29. Recommended V
AUX
Configuration
Power to the controller while operating in the self-bias or
DSS mode is provided by C
AUX
. Therefore, C
AUX
must be
sized such that a V
AUX
voltage greater than 7 V is
maintained while the outputs are switching and the
converter reaches regulation. Also, the V
AUX
discharge time
(from 11 V to 7 V) must be greater that the soft start charge
period to assure the converter turns ON.
The start-up circuit is rated at a maximum voltage of
150 V. If the device operates in the DSS mode, power
dissipation should be controlled to avoid exceeding the
maximum power dissipation of the controller.
13.8 mA
Disable
C
AUX
I
supply
V
AUX
I
AUX
To auxiliary supply
V
in
I
START