Datasheet

LT3641
13
3641fa
Figure 1. Lower Switching Frequency Occurs in High
Voltage Channel When Required On-Time Is Below 50ns
The high voltage nonsynchronous channel operates from
the V
IN
pin. The minimum V
IN
voltage to regulate output
voltage is:
V
IN(MIN)
=
V
OUT1
+ V
D
DC
MAX
⎛
⎝
⎜
⎞
⎠
⎟
− V
D
+ V
CE
Where V
D
is the forward voltage drop of the catch diode, V
CE
is the voltage drop of the internal NPN power switch, and
DC
MAX
is the maximum duty cycle (refer to the Switching
Frequency section). If V
IN
is below the calculated minimum
voltage, output will lose regulation.
The maximum V
IN
should not exceed the absolute
maximum rating. For fi xed frequency operation, the
maximum V
IN
is:
V
IN(MAX)
=
V
OUT1
+ V
D
DC
MIN
⎛
⎝
⎜
⎞
⎠
⎟
− V
D
+ V
CE
Note that the high voltage buck will still regulate at an
input voltage that exceeds V
IN(MAX)
(up to 42V). It will
continue to regulate through transients up to 55V for one
second. Note that the switching frequency will be reduced
once the on-time required to satisfy the above equation is
below 50ns (Figure 1).
V
IN2
Voltage Range
The low voltage synchronous channel operates from
the V
IN2
pin. The V
IN2
pin can be connected to either an
independent voltage supply or the high voltage channel
output for a two-stage power regulator. The V
IN2
voltage
range is 2.3V ~ 5.5V
The minimum V
IN2
voltage to regulate output voltage at
full frequency is:
V
IN2(MIN)
≈
V
OUT2
DC
MAX
Where DC
MAX
is the maximum duty cycle (refer to Switching
Frequency section). If V
IN2
is below the calculated minimum
voltage, the low voltage channel starts to skip oscillator
clock. In this case, the low voltage channel switching
frequency will no longer be the programmed frequency.
As the V
IN2
voltage further decreases, the top MOSFET
will remain on 100% duty cycle. In the case, the output
starts to fall out of regulation.
The maximum V
IN2
for fi xed frequency operation is:
V
IN2(MAX)
≈
V
OUT2
DC
MIN
Where DC
MIN
is the minimum duty cycle (refer to the
Switching Frequency section). For voltage that exceeds
V
IN2(MAX)
(up to 5.5V), the low voltage channel exhi-
bits pulse-skipping behavior, and the output ripple will
increase.
Inductor Selection
Inductor selection involves inductance, saturation current,
series resistance (DCR) and magnetic loss.
The inductance for the high voltage channel is:
L1= 1.7 •
V
OUT1
+ V
D
f
S
APPLICATIONS INFORMATION
200ns/DIV
SW1
10V/DIV
I
L1
0.5A/DIV
3641 F01
V
IN
= 30V
V
OUT1
= 3.3V/0.2A
R
T
SET = 2MHz