NCP1294
http://onsemi.com
10
Figure 8. The SYNC Pin Generates a Sync Pulse at
the Beginning of Each Switching Cycle.
CH2: GATE Pin, CH3: R
T
C
T
, CH4: SYNC Pin
Figure 9. Operation with External Sync.
CH2: SYNC Pin, CH3: GATE Pin, CH4: R
T
C
T
Pin
An external pulse signal can feed to the bidirectional
SYNC pin to synchronize the switch frequency. For reliable
operation, the sync frequency should be approximately 20%
higher than free running IC frequency. As show in Figure 9,
when the SYNC pin is triggered by an incoming signal, the
IC immediately discharges C
T
. The GATE signal is turned
on once the R
T
C
T
pin reaches the valley voltage. Because of
the steep falling edge, this valley voltage falls below the
regular 1.0 V threshold. However, the R
T
C
T
pin voltage is
then quickly raised by a clamp. When the R
T
C
T
pin reaches
the 0.95 V (typ) Valley Clamp Voltage, the clamp is
disconnected after a brief delay and C
T
is charged through
R
T
.
DESIGN GUIDELINES
Switch Frequency and Maximum Duty Cycle
Calculations
Oscillator timing capacitor, C
T
, is charged by V
REF
through R
T
and discharged by an internal current source.
During the discharge time, the internal clock signal sets the
Gate output to the low state, thus providing a user selectable
maximum duty cycle clamp. Charge and discharge times are
determined by following general formulas;
t
C
+ R
T
C
T
ln
ǒ
(V
REF
* V
VALLEY
)
(V
REF
* V
PEAK
)
Ǔ
t
d
+ R
T
C
T
ln
ǒ
(V
REF
* V
PEAK
* I
d
R
T
)
(V
REF
* V
VALLEY
* I
d
R
T
)
Ǔ
where:
t
C
= charging time;
t
d
= discharging time;
V
VALLEY
= valley voltage of the oscillator;
V
PEAK
= peak voltage of the oscillator.
Substituting in typical values for the parameters in the
above formulas, V
REF
= 3.3 V, V
VALLEY
= 1.0 V, V
PEAK
=
2.0 V, I
d
= 1.0 mA:
t
C
+ 0.57R
T
C
T
t
d
+ R
T
C
T
ln
ǒ
1.3 * 0.001R
T
2.3 * 0.001R
T
Ǔ
D
max
+
0.57
0.57 ) In
ǒ
1.3*0.001R
T
2.3*0.001R
T
Ǔ
It is noticed from the equation that for the oscillator to
function properly, R
T
has to be greater than 2.3 k.
Select RC for Feed Forward Ramp
If the line voltage is much greater than the FF pin Peak
Voltage, the charge current can be treated as a constant and
is equal to V
IN
/R. Therefore, the volt−second value is
determined by:
V
IN
T
ON
+ (V
COMP
* V
FF(d)
) R C
where:
V
COMP
= COMP pin voltage;
V
FF(d)
= FF pin discharge voltage.
As shown in the equation, the volt−second clamp is set by
the V
COMP
clamp voltage which is equal to 1.8 V. In
Forward or Flyback circuits, the volt−second clamp value is
designed to prevent transformers from saturation.
In a buck or forward converter, volt−second is equal to
V
IN
T
ON
+
ǒ
V
OUT
T
S
n
Ǔ
n = transformer turns ratio, which is a constant determined
by the regulated output voltage, switching period and
transformer turns ration (use 1.0 for buck converter). It is
interesting to notice from the aforementioned two equations
