IR3894 IRF [International Rectifier], IR3894 Datasheet - Page 25

• Manufacturer Part Number: IR3894
• Description: 12A HIGHLY INTERGRATED SUPLRBUCK
• Manufacturer: IRF [International Rectifier]

MINIMUM ON TIME CONSIDERATIONS
The minimum ON time is the shortest amount of time for
Ctrl FET to be reliably turned on. This is very critical
parameter for low duty cycle, high frequency applications.
Conventional approach limits the pulse width to prevent
noise, jitter and pulse skipping. This results to lower closed
loop bandwidth.
IR has developed a proprietary scheme to improve and
enhance minimum pulse width which utilizes the benefits
of voltage mode control scheme with higher switching
frequency, wider conversion ratio and higher closed loop
bandwidth, the latter results in reduction of output
capacitors. Any design or application using IR3894 must
ensure operation with a pulse width that is higher than this
minimum on‐time and preferably higher than 60 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 IR3894, the following condition
must be satisfied:
The minimum output voltage is limited by the reference
voltage and hence V
V
Therefore, at the maximum recommended input voltage of
21V and minimum output voltage, the converter should be
designed at a switching frequency that does not exceed
396 kHz. Conversely, for operation at the maximum
recommended operating frequency (1.65 MHz) and
minimum output voltage (0.5V). The input voltage (PVin)
should not exceed 5.05V, otherwise pulse skipping will
happen.
out(min)
= 0.5 V,

25
t

on
V


t
in
V

on
V
t
on

in
(min)
in
F
D
AUGUST 08, 2012 | DATA SHEET | Rev 3.1
(min)
out(min)
F

s

s
F
F


s
s


t


on
= 0.5 V. Therefore, for
V
V
V
t

in
t
out
0.5
60
on
V
on
in
V
V
out

(min)
(min)
(min)
out

out
ns
F
V
s
F



s

. 8
33
Single‐Input Voltage, Synchronous Buck Regulator
(5)
(6)
(4)
V/uS
(3)
- 25 -`
12A Highly Integrated SupIRBuck
MAXIMUM DUTY RATIO
A certain off‐time is specified for IR3894. This provides
an upper limit on the operating duty ratio at any given
switching frequency. The off‐time remains at a relatively
fixed ratio to switching period in low and mid frequency
range, while in high frequency range this ratio increases,
thus the lower the maximum duty ratio at which IR3894
can operate. Figure 21 shows a plot of the maximum duty
ratio vs. the switching frequency with built in input voltage
feed forward.
Figure 21: Maximum duty cycle vs. switching frequency.
IR3894
PD‐97745