Datasheet

LTC4156
43
4156f
Figure 11. Single-Input Overvoltage Protection
Figure 12. Dual-Input Positive and Negative Voltage Protection
Alternate Input Power Configurations
For applications requiring only a single input, the external
circuit required for overvoltage protection is considerably
simplified. Only a single N-channel MOSFET and resistor
are required for positive voltage protection, as shown in
Figure 11, and OVGCAP may be left unconnected. Applica-
tions using the USB On-The-Go step-up regulator
should
connect R1 to USBSNS and the gate of MN1 to USBGT.
Applications not using USB On-The-Go may use either the
USBSNS/USBGT pins or the WALLSNS/WALLGT pins. The
unused pins may be left unconnected.
For dual-input applications requiring reverse-voltage
protection, no additional power transistors are required.
The circuit in Figure 12 provides positive protection up to
the drain breakdown voltage rating of
MN3 and MN4 and
negative protection down to the drain breakdown volt-
age rating of MN1 and MN2. Q1 and Q2 are small-signal
transistors to protect the gate oxides of MN1 and MN2.
Note that it is necessary to orient the N-channel MOSFETs
with the drain connections common and the source/body
connections to the input connector and the V
BUS
pin.
Choosing the Inductor
The
LTC4156 is designed to operate with a 1µH inductor,
with core saturation, winding resistance, and thermal
rise characteristics appropriate for the application’s peak
currents. The inductor current ripple magnitude is ap-
proximately 400mA under normal conditions, resulting
in a peak inductor current 200mA higher than the aver-
age output current of the switching regulator. The aver-
age output current of the step-down regulator is
higher
than the average input current by the ratio V
BUS
/V
OUT,
neglecting efficiency losses. The LTC4156 can tolerate
transient excursions beyond the inductor’s core saturation
level, but the inductor current will increase rapidly to the
LTC4156’s peak current clamp as incremental inductance
tends toward zero. If an overload condition persists with
a small inductor, it is possible that the inductor could be
damaged
by its own resistive temperature rise.
APPLICATIONS INFORMATION
V
BUS
TO POWER
INPUT
OVGCAP
USBGT
USBSNS
WALLGT
LTC4156
WALLSNS
4156 F11
R1
MN1
V
BUS
TO USB
INPUT
TO WALL
INPUT
OVGCAP
USBGT
USBSNS
WALLGT
LTC4156
WALLSNS
4156 F12
MN1
R5
47k
R6
47k
C1
OPT
0.01µF
R3 5M
R4 5M
R1 3.6k
R2 3.6k
MN3
MN2 MN4
Q1
Q2
Q3
Q4