NCP1442, NCP1443, NCP1444, NCP1445
APPLICATIONS INFORMATION
THEORY OF OPERATION
Current Mode Control
V
CC
Oscillator
S
V
C
−
+
PWM
Comparator
Q
Power Switch
L
R
D1
V
SW
In Out
X5
Driver
C
O
R
LOAD
SUMMER
Slope Compensation
15 mW
Figure 26. Current Mode Control Scheme
The NCP144X family incorporates a current mode
control scheme, in which the PWM ramp signal is derived
from the power switch current. This ramp signal is compared
to the output of the error amplifier to control the on−time of
the power switch. The oscillator is used as a fixed−frequency
clock to ensure a constant operational frequency. The
resulting control scheme features several advantages over
conventional voltage mode control. First, derived directly
from the inductor, the ramp signal responds immediately to
line voltage changes. This eliminates the delay caused by the
output filter and error amplifier, which is commonly found
in voltage mode controllers. The second benefit comes from
inherent pulse−by−pulse current limiting by merely
clamping the peak switching current. Finally, since current
mode commands an output current rather than voltage, the
filter offers only a single pole to the feedback loop. This
allows both a simpler compensation and a higher
gain−bandwidth over a comparable voltage mode circuit.
Without discrediting its apparent merits, current mode
control comes with its own peculiar problems, mainly,
subharmonic oscillation at duty cycles over 50%. The
NCP144X family solves this problem by adopting a slope
compensation scheme in which a fixed ramp generated by
the oscillator is added to the current ramp. A proper slope
rate is provided to improve circuit stability without
sacrificing the advantages of current mode control.
Oscillator and Shutdown
Sync
Current
Ramp
V
SW
The oscillator is trimmed to guarantee frequency
accuracy. The output of the oscillator turns on the power
switch at a frequency of 280 kHz (NCP1442/3) or 560 kHz
(NCP1444/5), as shown in Figure 26. The power switch is
turned off by the output of the PWM Comparator.
A TTL−compatible sync input at the SS pin is capable of
syncing up to 1.8 times the base oscillator frequency. As
shown in Figure 27, in order to sync to a higher frequency,
a positive transition turns on the power switch before the
output of the oscillator goes high, thereby resetting the
oscillator. The sync operation allows multiple power
supplies to operate at the same frequency.
A sustained logic low at the SS pin will shut down the IC
and reduce the supply current.
An additional feature includes frequency shift to 20% of
the nominal frequency when either the NFB or FB pins
trigger the threshold. During power up, overload, or short
circuit conditions, the minimum switch on−time is limited
by the PWM comparator minimum pulse width. Extra
switch off−time reduces the minimum duty cycle to protect
external components and the IC itself.
As previously mentioned, this block also produces a ramp
for the slope compensation to improve regulator stability.
Error Amplifier
200 k
NFB
250 k
+
NCP1443/5
−
2.0 V
negative error−amp
1MW
120 pF
V
C
1.276 V
+
FB
Voltage
Clamp
C1
0.01
mF
R1
5 kW
−
NCP1442/4
positive error−amp
Figure 28. Error Amplifier Equivalent Circuit
Figure 27. Timing Diagram of Sync and Shutdown
For NCP1443/5, the NFB pin is internally referenced to
−2.475 V with approximately a 250 kW input impedance.
For NCP1442/4, the FB pin is directly connected to the
inverting input of the positive error amplifier, whose
non−inverting input is fed by the 1.276 V reference. Both
amplifiers are transconductance amplifiers with a high
output impedance of approximately 1.0 MW, as shown in
Figure 28. The V
C
pin is connected to the output of the error
amplifiers and is internally clamped between 0.5 V and
1.7 V. A typical connection at the V
C
pin includes a capacitor
in series with a resistor to ground, forming a pole/zero for
loop compensation.
An external shunt can be connected between the V
C
pin
and ground to reduce its clamp voltage. Consequently, the
current limit of the internal power transistor current is
reduced from its nominal value.
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