LT3640EFE LINER [Linear Technology], LT3640EFE Datasheet - Page 13

• Manufacturer Part Number: LT3640EFE
• Description: Dual Monolithic Buck Regulator with Power-On Reset and Watchdog Timer
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
The maximum V
mum rating. For fi xed frequency operation, the maximum
V
Note that the high voltage buck will still regulate at an input
voltage that exceeds V
switching frequency will be lowered to satisfy the equa-
tion (Figure 1).
Once the input voltage reaches 36V, an internal overvoltage
lockout (OVLO) circuit is triggered to disable switching ac-
tion (Figure 2). Without switching, the LT3640 can sustain
V
V
The low voltage synchronous channel operates from
the V
independent voltage supply or the high voltage channel
output for a two-stage power regulator.
In either confi guration, 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
Where DC
the Switching Frequency section). If V
IN
IN
IN2
V
V
is:
voltage transients up to 55V for one second.
Figure 1. Lower Switching Frequency Occurs in High
Voltage Channel When Required On-Time Is Below 50ns
IN MAX
IN MIN
Voltage Range
IN2
(
2
(
pin. The V
0.5A/DIV
10V/DIV
MAX
)
SW1
)
=
I
L1
≈
V
V
DC
⎛
⎝ ⎜
IN
OUT1
V
IN2
is the maximum duty cycle (refer to
IN
V
OUT
= 30V
MAX
OUT
should not exceed the absolute maxi-
IN2
= 3.3V/0.2A
DC
voltage to regulate output voltage is:
2
1
MIN
IN(MAX)
pin can be connected to either an
+
V
200ns/DIV
D
R
⎞
⎠ ⎟
T
SET = 2MHz
(up to 36V). However, the
−
V
D
+
V
IN2
CE
3640 F01
is below the
calculated minimum voltage, the output will fall out of
regulation.
The maximum V
Where DC
Switching Frequency section). For voltage that exceeds
V
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 inductor
current ripple determines the inductance. A reasonable cur-
rent ripple is around 30% of the maximum load current:
where I
tee suffi cient output current, peak inductor current must
be lower than the switch current limit (I
inductor current is:
where I
voltage channel, the top switch current limit is at least
2.4A at low duty cycles and decreases linearly to 1.8A at
DC = 0.8. Be sure to pick an inductor ripple current that
provides suffi cient maximum load current I
IN2(MAX)
ΔI
I
V
L PEAK
IN MAX
55V
(
L
2
= 0.3 • I
PK
(
OUT(MAX)
L(PEAK)
, 40V, 15V
20V/DIV
2A/DIV
)
MIN
(up to 5.5V), the low voltage channel exhi-
=
)
V
I
L1
IN
≈
I
Figure 2. V
OUT(MAX)
OUT MAX
is the minimum duty cycle (refer to the
is the peak inductor current. For the high
DC
V
IN2
is the maximum load current. To guaran-
OUT
(
MIN
for fi xed frequency operation is:
2
)
IN
+
Overvoltage Lockout
10μs/DIV
Δ
2
I
L
3640 F02
LIM
OUT(MAX)
LT3640
). The peak
13
. Once
3640p