參數(shù)資料
型號: LT4351CMS
廠商: LINEAR TECHNOLOGY CORP
元件分類: MOSFETs
英文描述: JACK RJ45 8POS/2PORT RTANG PCB
中文描述: 0.67 A BUF OR INV BASED MOSFET DRIVER, PDSO10
封裝: PLASTIC, MSOP-10
文件頁數(shù): 9/20頁
文件大?。?/td> 270K
代理商: LT4351CMS
LT4351
9
sn4351 4351fs
APPLICATIOU
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
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:
W
U
U
R
UV
I
UVHYST
R
V
UV
V
R
HYST
UV
FAULT
UV
2
1
2
=
=
where UV
HYST
is the desired undervoltage hysteresis at
the input. UV
FAULT
is the desired undervoltage trip voltage
at the input. V
UV
is the part undervoltage trip point (0.3V)
and I
HYSTUV
is the undervoltage hysteresis current (10
μ
A).
See Figure 1.
The divider on the OV pin is a straightforward resistive
divider (Figure 2):
R
OV
V
R
R
V
R R Divider Current
,
B
FAULT
OV
A
A
=
=
.
1
0 3
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.
R2
R1
UV
I
HYS
10
μ
A
V
300mV
V
IN
UV TURNING ON
UV TURNING OFF
R2
R1
UV
I
HYS
10
μ
A
V
300mV
4351 F01
V
IN
Figure 1
R
B
OV
R
A
V
300mV
4351 F02
V
IN
Figure 2
R2
R3
R1
OV
UV
4351 F03
V
IN
Figure 3
C1
R2A
R2B
R1
UV
4351 F04
V
IN
Figure 4
actively low. In the event of V
DD
collapse there still is an
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
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.
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.
OPERATIOU
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