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IR3841MPbF
Minimum on time Considerations
The minimum ON time is the shortest amount of
time for which the Control FET may be reliably
turned on, and this depends on the internal
timing delays. For the IR3841, the typical
minimum on-time is specified as 50 ns.
Any design or application using the IR3841 must
ensure operation with a pulse width that is higher
than this minimum on-time and preferably higher
than 100 ns. This is necessary for the circuit to
operate without jitter and pulse-skipping, which
can cause high inductor current ripple and high
output voltage ripple.
Maximum Duty Ratio Considerations
A fixed off-time of 200 ns maximum is specified
for the IR3841. This provides an upper limit on
the operating duty ratio at any given switching
frequency. It is clear, that higher the switching
frequency, the lower is the maximum duty ratio at
which the IR3841 can operate. To allow some
margin, the maximum operating duty ratio in any
application using the IR3841 should still
accommodate about 250 ns off-time. Fig 10.
shows a plot of the maximum duty ratio v/s the
switching frequency, with 250 ns off-time.
t on =
=
D
F s
V out
V in × F s
95
90
85
80
75
M a x Duty Cycle
In any application that uses the IR3841, the
following condition must be satisfied:
t on (min) ≤ t on
70
65
60
55
250
450
650
850
1050
1250
1450
1650
∴ t on (min) ≤
∴ V in × F s ≤
V out
V in × F s
V out
t on (min)
The minimum output voltage is limited by the
reference voltage and hence V out(min) = 0.7 V.
Therefore, for V out(min) = 0.7 V,
S w itching Frequency (kH z )
Fig. 10. Maximum duty cycle v/s switching
frequency.
∴ V in × F s ≤
V out (min)
t on (min)
∴ V in × F s ≤
0.7 V
100 ns
= 7 × 10 6 V/s
Therefore, at the maximum recommended input
voltage 16V and minimum output voltage, the
converter should be designed at a switching
frequency that does not exceed 440 kHz.
Conversely, for operation at the maximum
recommended operating frequency 1.65 MHz
and minimum output voltage, any voltage above
4.2 V may not be stepped down without pulse-
skipping.
06/18/09
16
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