NCP1442, NCP1443, NCP1444, NCP1445
The improved circuit does not require a regulated voltage
The dashed box contains the normal compensation
circuitry to limit the bandwidth of the error amplifier.
to operate properly. Unfortunately, a price must be paid for
this convenience in the overall efficiency of the circuit. The
designer should note that the input and output grounds are
no longer common. Also, the addition of the current sense
Resistors R2 and R3 form a voltage divider off of the V
SW
pin. In normal operation, V
looks similar to a square
SW
wave, and is dependent on the converter topology. Formulas
for calculating V in the boost and flyback topologies are
resistor, R , results in a considerable power loss which
SENSE
SW
increases with the duty cycle. Resistor R2 and capacitor C3
form a low−pass filter to remove noise.
given in the section “V Voltage Limit.” The voltage on
SW
V
SW
charges capacitor C3 when the switch is off, causing
the voltage at the V pin to shift upwards. When the switch
turns on, C3 discharges through R3, producing a negative
C
Subharmonic Oscillation
Subharmonic oscillation (SHM) is a problem found in
current−mode control systems, where instability results
when duty cycle exceeds 50%. SHM only occurs in
switching regulators with a continuous inductor current.
This instability is not harmful to the converter and usually
does not affect the output voltage regulation. SHM will
increase the radiated EM noise from the converter and can
cause, under certain circumstances, the inductor to emit
high−frequency audible noise.
SHM is an easily remedied problem. The rising slope of
the inductor current is supplemented with internal “slope
compensation” to prevent any duty cycle instability from
carrying through to the next switching cycle. In the
NCP144X family, slope compensation is added during the
entire switch on−time, typically in the amount of
180 mA/ms.
slope at the V pin. This negative slope provides the slope
compensation.
The amount of slope compensation added by this circuit
C
is
*(1*D)
R
f
3
SW
(1 * D)R A
E V
DI
DT
R C f
3 SW
3
SW ǒ
Ǔ
ǒ1 * e Ǔǒ
Ǔ
+ V
R )R
2 3
where:
DI/DT = the amount of slope compensation added (A/s);
= the voltage at the switch node when the transistor
V
SW
is turned off (V);
= the switching frequency, typically 280 kHz
f
SW
(NCP1442/3) or 560 kHz (NCP1444/5) (Hz);
D = the duty cycle;
R = 0.015 W, the value of the internal emitter resistor;
E
A = 5.0 V/V, the gain of the current sense amplifier.
V
In some cases, SHM can rear its ugly head despite the
presence of the onboard slope compensation. The simple
cure to this problem is more slope compensation to avoid the
unwanted oscillation. In that case, an external circuit, shown
in Figure 38, can be added to increase the amount of slope
compensation used. This circuit requires only a few
components and is “tacked on” to the compensation
network.
In selecting appropriate values for the slope compensation
network, the designer is advised to choose a convenient
capacitor, then select values for R2 and R3 such that the
amount of slope compensation added is 100 mA/ms. Then
R2 may be increased or decreased as necessary. Of course,
the series combination of R2 and R3 should be large enough
to avoid drawing excessive current from V . Additionally,
SW
to ensure that the control loop stability is improved, the time
constant formed by the additional components should be
chosen such that:
V
SW
V
SW
1 * D
R C
3 3
t
f
SW
V
C
Finally, it is worth mentioning that the added slope
compensation is a trade−off between duty cycle stability and
transient response. The more slope compensation a designer
adds, the slower the transient response will be, due to the
external circuitry interfering with the proper operation of the
error amplifier.
R1
R2
C1
C2
Soft−Start
Through the addition of an external circuit, a soft−start
function can be added to the NCP1442/3/4/5 family of
components. Soft−start circuitry prevents the V pin from
C
slamming high during startup, thereby inhibiting the
inductor current from rising at a high slope.
C3
R3
This circuit, shown in Figure 39, requires a minimum
number of components and allows the soft−start circuitry to
activate any time the SS pin is used to restart the converter.
Figure 38. Technique for Increasing Slope
Compensation
http://onsemi.com
15
ONSEMI [ ONSEMI ]
