Datasheet

LT4351
9
4351fd
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 cannot do.
A resistive divider from V
IN
to UV and one from V
IN
to OV
are the usual way of setting the FAULT thresholds. For UV
the resistor values are set by:
R2 =
UV
HYST
I
UVHYST
R1=
V
UV
UV
FAULT
– V
UV
• R2
where UV
HYST
is the desired undervoltage hysteresis at
the input. UV
FAULT
is the desired undervoltage trip volt-
APPLICATIONS INFORMATION
Figure 1
age at the input. V
UV
is the part undervoltage trip point
(0.3V) and I
HYSTUV
is the undervoltage hysteresis current
(10µA). See Figure 1.
The divider on the OV pin is a straightforward resistive
divider (Figure 2):
R
B
=
OV
FAULT
V
OV
– 1






R
A
R
A
=
0.3V
R
A
,R
B
Divider Current
where OV
FAULT
is the desired overvoltage trip point at the
input and V
OV
is the OV pin threshold (0.3V). The OV pin
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.
The on-chip boost regulator uses a constant off-time
control scheme. When V
DD
is below the regulation trip
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 and the inductor’s current flows through the external
diode to charge up the V
DD
capacitor. If V
DD
is still too low,
the switch turns on again after a fixed off-time of 600ns.
OPERATION
The boost regulator regulates V
DD
to approximately 10.7V
above V
IN
When V
DD
is above this level, the SW transistor
turn-on is disabled. When V
DD
falls below this level by the
hysteresis level, the SW transistor is allowed to turn on.
There is approximately 0.15V of hysteresis.
Figure 2 Figure 3 Figure 4
R2
R1
UV
I
HYS
10µA
V
UV
300mV
V
IN
UV TURNING ON UV TURNING OFF
R2
R1
UV
I
HYS
10µA
V
UV
300mV
4351 F01
V
IN
R
B
R
A
OV
V
OV
300mV
4351 F02
V
IN
R2
R3
R1
OV
UV
4351 F03
V
IN
C1
R2A
R2B
R1
UV
4351 F04
V
IN