mc44603 ON Semiconductor, mc44603 Datasheet - Page 17

• Manufacturer Part Number: mc44603
• Description: Mixed Frequency Mode Greenline Pwm Controller
• Manufacturer: ON Semiconductor

Maximum Duty Cycle and Soft–Start Control
80% by utilizing the D max and soft–start control. As depicted
in Figure 42, the Pin 11 voltage is compared to the oscillator
sawtooth.
voltage can easily be set by connecting a resistor to this pin.
from 0 to its maximum value progressively (refer to Figure
44), thereby, implementing a soft–start. The soft–start
capacitor is discharged internally when the V CC (Pin 1)
voltage drops below 9.0 V.
internal zener diode clamps the Pin 11 voltage to a value V Z
that is higher than the oscillator peak value, disabling
soft–start and maximum duty cycle limitation.
Foldback
used to reduce the maximum V CS value, providing foldback
protection. The foldback arrangement is a programmable
peak current limitation.
current becomes higher and V CS increases until it reaches its
maximum value (normally, V CS max = 1.0 V).
unable to supply enough energy to maintain the output
voltages in regulation. Consequently, the decreasing output
can be applied to Pin 5, in order to limit the maximum peak
current. In this way, the well known foldback characteristic
can be obtained (refer to Figure 45).
Pin 11
MOTOROLA ANALOG IC DEVICE DATA
Voltage
Maximum duty cycle can be limited to values less than
Using the internal current source (0.4 I ref ), the Pin 11
If a capacitor is connected to Pin 11, the voltage increases
If no external component is connected to Pin 11, an
As depicted in Fgure 32, the foldback input (Pin 5) can be
If the output load is increased, the required converter peak
Then, if the output load keeps on increasing, the system is
D max
RI
Soft–Start
Capacitor
Figure 44. Different Possible Uses of Pin 11
R Connected to Pin 11
I = 0.4 I ref
Figure 43. Maximum Duty Cycle Control
11
Figure 42. D max and Soft–Start
D Z
V ref
2.4 V
0.4 I ref
V Z
V OSC
C Dmax
Oscillator
C
Control
Output
D max
V Z
RI
Output
Drive
C // R
τ = RC
Pin 11
V CT
(Pin 10)
MC44603
NOTE: Foldback is disabled by connecting Pin 5 to V CC .
Overvoltage Protection
that compares the Pin 6 voltage to V ref (2.5 V) (refer to
Figure 46).
comparator noninverting input voltage is nearly equal to:
The comparator output is high when:
overvoltages that last at least 2.0 µs.
output, becomes high. As this level is brought back to the
input through an OR gate, V OVP out remains high (disabling
the IC output) until V ref is disabled.
detected, the output is disabled until V CC is removed and
then re–applied.
in order to limit the circuit startup consumption.
regulator has started to allow the reference V ref to stabilize.
V CC level can be changed.
External
Resistor
The overvoltage arrangement consists of a comparator
If no external component is connected to Pin 6, the
A delay latch (2.0 µs) is incorporated in order to sense
If this condition is achieved, V OVP out , the delay latch
Consequently, when an overvoltage longer than 2.0 µs is
The V CC is connected after V ref has reached steady state
The overvoltage section is enabled 5.0 µs after the
By connecting an external resistor to Pin 6, the threshold
V disable2
Nominal
V CC
V OVP
V O
V out
6
Figure 45. Foldback Characteristic
Figure 46. Overvoltage Protection
11.6 k
0
V CC
11.6 k
2.0 k
T
2.0 k
2.5 V
W
11.6 k
)
2.5 V
(V ref )
à
Enable
W
2.0 k
C OVLO
Delay
Sequence Initiated
V CC
2.0 k
V ref
Out
W
New Startup
In
)
W
τ
w
W
2.0 k
5.0 µs
17 V
x V CC
W
the Output is Disabled)
(If V OVP out = 1.0,
I pk max
In
Delay
w
2.0 µs
x V CC
τ
2.5 V
Out
Overload
V OVP out
I out
17