ir3840 International Rectifier Corp., ir3840 Datasheet - Page 16

• Manufacturer Part Number: ir3840
• Description: Integrated 12a Synchronous Buck Regulator
• Manufacturer: International Rectifier Corp.

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 IR3840, the typical
minimum on-time is specified as 50 ns.
Any design or application using the IR3840 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.
In any application that uses the IR3840, the
following condition must be satisfied:
The minimum output voltage is limited by the
reference voltage and hence V
Therefore, for V
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.
02/09/09
∴
∴
V
∴
∴
in
t
V
on
V
t
in
×
on
t
(min)
in
on
F
×
(min)
×
s
out(min)
F
=
=
F
s
≤
≤
V
s
F
D
≤
≤
in
t
V
V
s
≤
on
t
V
out
100
out
on
×
0.7
= 0.7 V,
t
in
V
on
V
(min)
(min)
F
out
×
out
(min)
s
ns
V
F
s
=
7 ×
out(min)
10
6
V/s
= 0.7 V.
Maximum Duty Ratio Considerations
A fixed off-time of 200 ns maximum is specified
for the IR3840. 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 IR3840 can operate. To allow some
margin, the maximum operating duty ratio in any
application
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.
Fig. 10. Maximum duty cycle v/s switching
95
90
85
80
75
70
65
60
55
250
frequency.
450
using
650
S w itching Frequency (kH z )
M a x Duty Cycle
the
850
IR3840MPbF
IR3840
1050
1250
should
1450
still
16
1650