NCP1212
http://onsemi.com
11
DETAILED OPERATING DESCRIPTIONS
INTRODUCTION
The NCP1212 implements a standard current mode
architecture where the switch−off time is dictated by the
peak current setpoint. This device represents an ideal
candidate where low external part−count is the key system
requirement. Additionally, the device provides extensive
value−added functions, Soft−Start, Brownout Detect, etc.,
that can be applied to low−cost AC−DC adaptor
applications. The NCP1212 incorporates all the necessary
functions normally needed in UC384X based power supply
systems: Oscillator section, PWM Latch section, Current
Sense section, Brownout Detect protection, Soft−Start and
Maximum Duty Cycle Selection. With all those functions,
this device becomes a good alternative to UC384X that can
help to improve both performance and system cost. Also, the
innovative Maximum Duty Cycle Selection feature allows
the device applied to both forward and fly−back mode
configurations. Detailed functions of individual internal
blocks are described in below and a simplified functional
block diagram is shown in Figure 2.
Oscillator Section
The oscillator frequency is programmed by the capacitor
connected to C
T
pin. The capacitor is charged by a constant
current source to 3.8 V and 2.5 V for 82% and 48%
maximum Duty Cycle condition respectively. Once the
selected voltage is reached, C
T
is then discharged by another
constant current source down to 1.0 V and this charging and
discharging action will carry on perpetually. Desirable
switching frequency can be selected by choosing proper
value of timing capacitor, C
T
. The C
T
pin waveform is
shown in Figure 32.
Figure 32. C
T
Pin Waveform for Oscillator
3.8 V
2.5 V
1.0 V
48%
48%
82%
PWM Latch Section
NCP1212 works in current mode. The power switch
current is converted to a positive voltage by inserting a
sensing resistor R
sense
between the power switch source and
the ground. The power switch peak current is compared with
the level shifted control input voltage on a cycle−by−cycle
basis. Figure 27 illustrated the internal blocks of the
function. The PWM latch is initialized by the Oscillator set
signal and is terminated by the current sense comparator
when the current exceeds the value dictated by the control
input or current limit level. The current sense Comparator
Latch configuration used ensures that only a single pulse
appears at the output during any given oscillator cycle.
Figure 33. PWM Latch Function
FB
+5 V
3 R
R
9 K
300 ns
LEB
S
Q
Q
Current
Limit
+
−
Output
+
−
Output
+
−
1 V
Oscillator
Reset
Set
R
F/F
DRV
CS
1
2
3
4
Rsense
PWM Control
Totem Pole Driver
