Datasheet

LT3640
13
3640f
Figure 1. Lower Switching Frequency Occurs in High
Voltage Channel When Required On-Time Is Below 50ns
The maximum V
IN
should not exceed the absolute maxi-
mum rating. For fixed frequency operation, the maximum
V
IN
is:
V
V V
DC
V V
IN MAX
OUT D
MIN
D CE( )
=
+






− +
1
Note that the high voltage buck will still regulate at an input
voltage that exceeds V
IN(MAX)
(up to 35V). However, the
switching frequency will be lowered to satisfy the equa-
tion (Figure 1).
Once the input voltage reaches 36.5V, an internal overvoltage
lockout (OVLO) circuit is triggered to disable switching ac-
tion (Figure 2). Without switching, the LT3640 can sustain
V
IN
voltage transients up to 55V for one second.
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.
In either configuration, if the high voltage channel is over-
loaded and pulled out of regulation, the low voltage channel
will be disabled. The SS2 pin will be discharged as well.
The minimum V
IN2
voltage to regulate output voltage is:
V
V
DC
IN MIN
OUT
MAX
2
2
( )
≈
Where DC
MAX
is the maximum duty cycle (refer to
the Switching Frequency section). If V
IN2
is below the
calculated minimum voltage, the output will fall out of
regulation.
The maximum V
IN2
for fixed frequency operation is:
V
V
DC
IN MAX
OUT
MIN
2
2
( )
≈
Where DC
MIN
is the minimum duty cycle (refer to the Switch-
ing Frequency section). For voltage that exceeds V
IN2(MAX)
(up to 5.5V), the low voltage channel exhi-bits pulse-skip-
ping 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:
L
V V
f
OUT D
S
1 1 7
1
=
+
. •
where V
OUT1
is high voltage channel output voltage, V
D
is the forward voltage drop of the catch diode, and f
S
is
the switching frequency. For example, 3.3µH is a reason-
able inductance for a 3.3V output with 2MHz switching
frequency.
Once the inductance is selected, the inductor current ripple
and peak current can be calculated:
∆I
V V
I f
V V
V
L
OUT D
L S
OUT D
IN
1
1
1
1
1=
+ +






( )
•
• –
I I
I
L PEAK OUT MAX
L
( ) ( )
= +
∆
2
Figure 2. V
IN
Overvoltage Lockout
applicaTions inForMaTion
200ns/DIV
SW1
10V/DIV
I
L1
0.5A/DIV
3640 F01
V
IN
= 30V
V
OUT1
= 3.3V/0.2A
R
T
SET = 2MHz
10µs/DIV
V
IN
20V/DIV
55V
PK
, 40V, 15V
I
L1
2A/DIV
3640 F02