lt3692a Linear Technology Corporation, lt3692a Datasheet - Page 25

• Manufacturer Part Number: lt3692a
• Description: Monolithic Dual Tracking 3.5a Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

Single Step Down:
2-Stage Step-Down:
2-Stage Step-Down Multi-Frequency:
R
In addition, R
on Channel 2 to 2A, which reduces inductor size and catch
diode requirements.
Shorted and Reverse Input Protection
If the inductor is chosen so that it won’t saturate excessively,
an LT3692A step-down regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT3692A is absent. This may occur in battery charging
applications or in battery back-up systems where a battery
or some other supply is diode OR-ed with the LT3692A’s
output. If the V
pin is held high (either by a logic signal or because it is
tied to V
its quiescent current through its SW pin. This is fine if your
APPLICATIONS INFORMATION
Frequency (Hz) =
L1 =
L2 =
Frequency (Hz) =
L1 =
L2 =
DIV
L1=
L2 =
= 100k, FREQ1 = 550kHz, FREQ2 = 2200kHz.
(
(
(
(
36V – 3.3
36V – 3.3
36V – 1.2
3.3 – 1.2
36V • 350kHz
36V • 750kHz
36V • 350kHz
3.3 • 750kHz
(
IN
(
36V – 3.3
3.3 • 2200K Hz
3.3 – 1.2
36V • 550kHz
), then the LT3692A’s internal circuitry will pull
ILIM2
IN1/2
)
)
)
)
• 1.2
• 3.3
• 3.3
• 1.2
= 33.2k reduces the peak current limit
)
36V – 0.4 + 0.6
)
35V – 0.4 + 0.6
• 1.2
pin is allowed to float and the SHDN
• 3.3
1.2 + 0.6
3.3 + 0.6
≥ 1.0μH
≥ 8.5μH
≥ 4.0μH
≥ 3.3μH
≥ 0.47μH
≥ 5.4μH
•
•
140ns
140ns
1
1
≅ 750kHz
≅ 350kHz
system can tolerate a few mA in this state. If you ground
the SHDN pin, the SW pin current will drop to essentially
zero. However, if the V
is held high, then parasitic diodes inside the LT3692A can
pull large currents from the output through the SW pin
and the V
only when the input voltage is present and that protects
against a shorted or reversed input.
Figure 17. Diode D4 Prevents a Shorted Input from Discharging a
Backup Battery Tied to the Output
Figure 18. Subtracting the Current when the Switch Is On (18a)
from the Current when the Switch Is Off (18b) Reveals the Path of
the High Frequency Switching Current (18c). Keep this Loop Small.
The Voltage on the SW and BST Traces will Also Be Switched; Keep
These Traces as Short as Possible. Finally, Make Sure the Circuit
Is Shielded with a Local Ground Plane
V
IN1/2
IN1/2
PARASITIC DIODE
D4
pin. Figure 17 shows a circuit that will run
V
IN
V
V
V
IN
IN
IN
IN
LT3692A SW
LT3692A SW
LT3692A SW
LT3692A
GND
GND
GND
pin is grounded while the output
(18b)
(18a)
(18c)
SW
3692a F18
LT3692A
3692a F17
V
OUT1/2
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3692af