lt4351 Linear Technology Corporation, lt4351 Datasheet - Page 9

• Manufacturer Part Number: lt4351
• Description: Mosfet Diode-or Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
OPERATIO
actively low. In the event of V
active pull-down (though of lesser strength) of GATE
powered from OUT, guaranteeing turn off.
The on-chip boost regulator uses a constant off-time
control scheme. When V
voltage, the switch turns on after a 600ns off-time. When
the switch turns on current ramps up in the inductor until
the current limit is reached (450mA). The switch turns off
Setting Fault Thresholds
The gate drive amplifier implements the ideal diode func-
tion. The fault comparators (UV and OV) prevent out of
range input voltages from affecting the output by disabling
the amplifier during these conditions. Think of the UV and
OV as gating the ideal diode function, something a regular
diode can’t do.
A resistive divider from V
are the usual way of setting the FAULT thresholds. For UV
the resistor values are set by:
where UV
the input. UV
UV TURNING ON
V
IN
R
R
R2
R1
2
1
UV
UV
I
UV
UVHYST
HYST
I
10 A
Figure 1
HYS
FAULT
HYST
V
300mV
UV
FAULT
V
UV
is the desired undervoltage hysteresis at
–
U
UV TURNING OFF
is the desired undervoltage trip voltage
V
U
V
IN
UV
R2
R1
4351 F01
UV
•
IN
R
DD
U
2
to UV and one from V
I
10 A
HYS
is below the regulation trip
V
300mV
DD
UV
collapse there still is an
W
Figure 2
U
V
IN
IN
R
OV
R
4351 F02
to OV
B
A
V
300mV
OV
at the input. V
and I
See Figure 1.
The divider on the OV pin is a straightforward resistive
divider (Figure 2):
where OV
input and V
has 7mV of voltage hysteresis at room.
It is possible to do both dividers together using only three
resistors though with more interdependence in compo-
nents (Figure 3). The input bias current for UV and OV is
less than 200nA, so keep resistor values less than 10k.
and the inductor’s current flows through the external
diode to charge up the V
the switch turns on again after a fixed off-time of 600ns.
The boost regulator regulates V
above V
turn-on is disabled. When V
hysteresis level, the SW transistor is allowed to turn on.
There is approximately 0.15V of hysteresis.
R
R
B
A
HYSTUV
IN
R R Divider Current
FAULT
. When V
OV
A
OV
is the undervoltage hysteresis current (10 A).
,
V
FAULT
OV
Figure 3
UV
B
is the OV pin threshold (0.3V). The OV pin
is the desired overvoltage trip point at the
V
is the part undervoltage trip point (0.3V)
IN
0 3
R3
R2
R1
DD
4351 F03
–
.
1
is above this level, the SW transistor
V
UV
OV
DD
R
A
capacitor. If V
DD
falls below this level by the
DD
to approximately 10.7V
DD
is still too low,
Figure 4
LT4351
C1
V
IN
R2B
R2A
R1
4351 F04
sn4351 4351fs
UV
9