AN8011S
13
SDH00002BEB
■ Application Notes (continued)
[5] Triangular wave oscillation circuit
• Oscillation frequency setting method
The waveform of triangular wave oscillation is obtained by charging and discharging of the constant current I
O
from the external timing capacitor C
T
which is connected to CT terminal. The constant current is set by the externally
attached timing resistor R
T
.
The peak value of the wave V
CTH
and the trough
value of the wave V
CTL
are fixed at approximately 1.4 V
typical and 0.4 V typical respectively.
The oscillation frequency f
OSC
is obtained by the
following formula;
f
OSC
=
1
=
I
O
(typ.)
t
1
+ t
2
2 × C
T
× (V
CTH
− V
CHL
)
Where I
O
= 2 ×
V
RT
= 2 ×
0.67
R
T
R
T
Because V
CTH
− V
CTL
= 1 V
f
OSC
=
0.67
[Hz]
(typ.)
C
T
× R
T
The output frequency f
OUT
is equal to f
OSC
since it is PWM-controlled.
[6] Dead-time (maximum duty) setting method
The setting of the dead-time is conducted by adjusting the DTC terminal voltage V
DTC
as shown in figure 6. Since
the DTC terminal provides a constant current output through the resistor R
T
, V
DTC
is adjusted by attaching the external
resistor R
DTC
.
The output duty ratio Du and the DTC terminal voltage V
DTC
are expressed by the following formula. When the
oscillation frequency f
OSC
is 200 kHz, the output duty ratio is 0% at V
DTC
= 0.42 V, and 100% at V
DTC
= 1.35 V.
Pay attention to the peak value and the trough value of triangular wave because the overshoot and undershoot
voltages depend on the frequency.
t
1
T
Figure 5. Triangular wave oscillation waveform
V
CTH
= 1.4 V typ.
V
CTL
= 0.4 V typ.
t
2
Charging
Discharging
t
OFF
V
CTH
V
DTCDTC waveform
Out waveform
C
T
waveform
V
CTL
t
ON
Off OffOn
V
REF
V
RT
I
DTC
Figure 6. Dead-time setting method
R
DTC
C
DTC
DTC
I
DTC
=
CT
R
T
PWM
=
V
CTH
−V
DTC
V
CTH
−V
CTL
= V
RT
×
× 1.04 [V]
V
DTC
= I
DTC
× R
DTC
R
DTC
100 [%]
R
T
× 1.04 [A]
FB
×
Du
=
t
ON
t
ON
+ t
OFF
100 [%]
×
(typ.)
(typ.)
ex.) When f
OSC
= 200 kHz (R
T
= 20 kΩ, C
T
= 150 pF),
V
CTH
≈ 1.4 V (typ.) V
RT
≈0.67 V (typ.)
V
CTL
≈ 0.4 V (typ.) I
DTL
≈ 35 µA (typ.)
When the capacitor C
DTC
is added in parallel with the external resistor R
DTC
, the soft start function gradually extends
the on-period of the output pulse when the power supply is turned on. This prevents the overshoot of the DC-DC
converter output.
