lt4351 Linear Technology Corporation, lt4351 Datasheet - Page 14

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

APPLICATIO S I FOR ATIO
LT4351
Start-Up Considerations
There is no inherent shutdown in the part. As V
the boost regulator starts at about 0.85V and becomes fully
operational by 1.1V. The undervoltage and overvoltage
comparators become accurate by 1.2V. The gate drive
amplifier keeps GATE low during this period with either a
passive pull-down, a weak active pull-down if OUT is greater
than 0.8V or with the full gate drive sink if V
Once V
operates normally. The UV and OV pins will control the
enabling of the gate driver and once enabled, the V
OUT voltage controls MOSFET turn on.
If V
the MOSFET, the GATE pin tracks with the V
until it reaches either the gate clamp voltage or the
compliance of the gate driver. If V
or OUT, the GATE pin actively sinks low.
Power Dissipation
The internal power dissipation of the LT4351 is comprised
of the following four major components: DC power dissi-
pation from V
dissipation in the boost switch including the base drive,
and dynamic power dissipation due to current used to
charge and discharge the MOSFETs. The DC components
are:
14
P
P
DD
DCVIN
DCVDD
is still being charged when the gate driver turns on
IN
is greater than 1.2V and V
0.30
0.20
0.15
0.10
0.25
= I
= I
VIN
0
VDD
IN
• V
, DC power dissipation from V
• V
Figure 11. P
IN
U
5
DD
U
V
IN
10
(V)
L = 10 H
BOOST(MAX)
DD
W
15
DD
is present without V
L = 4.7 H
is up, the part then
4351 F11
DD
20
is above 2.2V.
IN
DD
U
ramps up,
increase
DD
, the
IN
to
IN
Figure 11 shows the internal dissipation of the boost
regulator as a function of V
represents the worst-case condition with the regulator on
all the time, which does not occur in normal practice.
Since the boost regulator supplies current for V
current is the V
current to charge the gate. For a gate charge of 50nC at a
10kHz rate, this adds 0.5mA of current. The power dissi-
pated by the boost regulator to supply the 4mA is shown
in Figure 12, representing a more typical situation.
Finally, the gate driver dissipates power internally when
charging and discharging the gate of the MOSFETs. This
power depends on the input capacitance of the MOSFETs
and the frequency of charge and discharge. The power
associated with this can be approximated by:
where Q
MOSFET to the clamp voltage (7.4V) and f
quency at which the gate is charged and discharged.
Normally f
low. Figure 13 shows P
1kHz rate.
Total power dissipation is the sum of all of P
P
power dissipation of a typical application at steady state.
BOOST
P
GATE
and P
G
0.025
0.015
0.010
0.005
0.020
G
is the required gate charge to charge the
f
is low and the resulting power would be very
G
0
GATE
•
L = 4.7 H
DD
V
DD
supply current (3.5mA) plus the average
. Figure 14 is representative of the total
Figure 12. P
•
5
Q
G
GATE
•
IN
V
IN
– 1
10
and inductor value. Figure 11
(V)
BOOST(TYP)
for a 50nC gate charge at a
V
16
IN
15
4351 F12
DCVIN
20
G
is the fre-
, P
sn4351 4351fs
DD
DCVDD
, the
,