MB3769A
Fig. 5 - MB3769A Equivalent Circuit Over Current Detection Comparator
V
REF
To PWM
Comp.
+IN
(C)
-IN
(C)
Protection element
3. DTC: Dead Time Control (Soft-Start and Quick Shutdown)
The dead time control terminal and the error amplifier output are connected to the PWM comparator.
The maximum duty cycle for V
DTC
(voltage applied to pin 4) is obtained from the following formula (approximate value at low
frequency):
Duty Cycle = (3.5 - V
DTC
) x 50 (%) [0%
≤
duty cycle
≤
D
MAX
(80%)]
The dead time control terminal is used to provide soft start.
In Figure 6, the DTC terminal is connected to the V
REF
terminal through R and C. Because capacitor C does not charge
instantaneously when the power is turned on, the output transistor is kept turned off. The DTC input voltage and the output pulse
width increase gradually according to the RC time constant so that the control system operates safely.
Fig. 6 - MB3769A Soft Start Function
V
REF
C
DTC
R
Soft Start
R
2
Soft Start + DTC
C
R
1
DTC
V
REF
The quick shutdown function prevents soft start malfunction when the power is turned off and on quickly. After the power is shut
down, soft start is disabled because the DTC terminal has low electric potential from the beginning if the power is turned on
again before the capacitor is discharged. The MB3769A prevents this by turning on the discharge transistor to quickly discharge
the capacitor in the stand-by mode.
11
FUJITSU [ FUJITSU COMPONENT LIMITED. ]
