LT4351_15 Datasheet, PDF (10/20 Pages) Linear Technology – MOSFET Diode-OR Controller

LT4351
APPLICATIONS INFORMATION
In that case, the resistor values are set by:
R3 = ^UVHYST
I_UVHYST
R2 =
V_UV
– ^UVFAULT
OV_FAULT
• V_OV
• R3
UV_FAULT – V_UV
( ) ^R1=
V_OV • UV_FAULT
• R3
OV_FAULT • UV_FAULT – V_UV
Hysteresis helps prevent erratic behavior due to the noise
on V_IN. Two of the most common noise sources are: V_IN
dipping when the MOSFETs first turn on and draw down
the voltage on the V_IN capacitors, and the boost regulator
switch turning on and drawing current from the V_IN ca-
pacitors. Use low ESR capacitors for V_IN and OUT filtering.
Note that because the UV pin uses current hysteresis,
placing a capacitor on UV to ground to filter noise will
reduce the effective hysteresis. Filtering can be achieved
by splitting the R2 resistor, as shown in Figure 4.
To defeat undervoltage fault detection, the UV pin should
be tied higher than 0.33V. UV can be tied to V_IN provided
V_IN < 9V. Overvoltage fault detection can be defeated by
grounding the OV pin. Do not exceed V_IN.
INPUT
REFERRED
OV_FAULT
UV_FAULT + UV_HYST
UV_FAULT
OVERVOLTAGE FAULT:
GATE LOW
OVERVOLTAGE FILTERED FAULT
GATE CONTROLLED
BY V_IN – V_OUT
OV
UV
REFERRED REFERRED
V_UV = 0.33V
V_UV = 0.3V
V_OV > 0.3V V_UV < 0.3V
UNDERVOLTAGE HYSTERESIS
UNDERVOLTAGE FAULT:
GATE LOW
V_OV = 0.3V
4351 F05
Figure 5. Graphical Representation of the UV and OV Functions
External Shutdown
To externally turn off the MOSFETs, such as to disable the
supply, use an open-collector transistor pulling down on
the UV pin. Note this will not turn off the boost regulator
which will continue to operate.
10
Boost Regulator
The boost regulator will start working as soon as V_IN is
greater than 0.85V. The regulator will supply all the cur-
rent for the gate drive amplifier. While the amplifier itself
requires only about 3mA, larger current pulses are required
when charging the MOSFET gate. The reservoir capacitor
on V_DD will provide this current (Figure 6).
V_IN
LT4351
SW
QSW
GND
L1
D1
V_DD
D2
C_DD
4351 F06
Figure 6
The regulator performance is relatively insensitive to the
inductor value. The inductor value does control the fre-
quency of operation. A 4.7µH inductor is recommended
for V_IN voltages less than 10V and 10µH for V_IN voltages
greater than 10V. Several inductors that work well with
the LT4351 are listed in Table 1. Many different sizes and
shapes are available. Consult each manufacturer for more
detailed information and for their entire selection of related
parts. The switching frequency for the boost regulator is
around 1MHz so ferrite core inductors should be used
to obtain the best efficiency. The inductor must handle a
peak current of 0.7A minimum and have a DC resistance
of 0.5Ω or less. Shielded inductors are recommended to
reduce the noise due to inductive switching.
Table 1. Recommended Inductors
PART NUMBER
IND (µH) DCR (mΩ)
LPS3314-472ML
4.7
175
LPS4012-103ML
10
350
744029004
744042100
4.7
200
10
150
SD3112-4R7-R
4.7
246
SD3118-100-R
10
295
VENDOR
Coilcraft
847-639-6400
www.coilcraft.com
Würth Elektronik
www.we-online.com
Coiltronics
www.coiltronics.com
4351fd