LT3758 LINER [Linear Technology], LT3758 Datasheet - Page 12

• Manufacturer Part Number: LT3758
• Description: High Input Voltage, Boost, Flyback, SEPIC and Inverting Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3758
INTV
An internal, low dropout (LDO) voltage regulator produces
the 7.2V INTV
shown in Figure 1. The LT3758 contains an undervoltage
lockout comparator A8 and an overvoltage lockout com-
parator A9 for the INTV
(UV) threshold is 4.5V (typical), with 0.5V hysteresis, to
ensure that the MOSFETs have suffi cient gate drive voltage
before turning on. The logic circuitry within the LT3758 is
also powered from the internal INTV
The INTV
(typical) to protect the gate of the power MOSFET. When
INTV
age threshold, the GATE pin will be forced to GND and the
soft-start operation will be triggered.
The INTV
mediately adjacent to the IC pins with a minimum of 4.7μF
ceramic capacitor. Good bypassing is necessary to supply
the high transient currents required by the MOSFET gate
driver.
In an actual application, most of the IC supply current is
used to drive the gate capacitance of the power MOSFET.
The on-chip power dissipation can be a signifi cant concern
when a large power MOSFET is being driven at a high
frequency and the V
limit the power dissipation through selection of MOSFET
and/or operating frequency so the LT3758 does not exceed
its maximum junction temperature rating. The junction
temperature T
equations:
T
θ
P
I
I
f = switching frequency
Q
12
Q
DRIVE
A
JA
IC
G
= V
T
= V
= ambient temperature
= power MOSFET total gate charge
= IC power consumption
J
= junction-to-ambient thermal resistance
CC
CC
= T
IN
IN
= average gate drive current = f • Q
Regulator Bypassing and Operation
is below the UV threshold, or above the overvolt-
• (I
operation I
A
CC
CC
+ P
Q
regulator must be bypassed to ground im-
overvoltage threshold is set to be 17.5V
+ I
IC
CC
J
• θ
DRIVE
supply which powers the gate driver, as
can be estimated using the following
JA
Q
IN
= 1.6mA
)
CC
voltage is high. It is important to
supply. The INTV
CC
supply.
CC
G
undervoltage
The LT3758 uses packages with an Exposed Pad for en-
hanced thermal conduction. With proper soldering to the
Exposed Pad on the underside of the package and a full
copper plane underneath the device, thermal resistance
(θ
for the MSE package. For an ambient board temperature of
T
the maximum I
be calculated as:
The LT3758 has an internal INTV
function to protect the IC from excessive on-chip power
dissipation. The I
increases (see the INTV
graph in the Typical Performance Characteristics section).
If I
and may trigger the soft-start.
Based on the preceding equation and the INTV
Output Current vs V
maximum MOSFET gate charge the LT3758 can drive at
a given V
Q
4.7V INTV
A
G
JA
DRIVE
I
= 70°C and maximum junction temperature of 125°C,
Figure 2. Recommended Maximum Q
Frequencies to Ensure INTV
vs V
DRIVE MAX
) will be about 43°C/W for the DD package and 40°C/W
IN
reaches the current limit, INTV
(
IN
at different frequencies to guarantee a minimum
CC
140
120
100
and switch frequency. A plot of the maximum
80
60
40
20
0
is shown in Figure 2.
)
1
=
DRIVE
(
DRIVE
(
θ
T
JA
J
IN
(I
−
•
DRIVE(MAX)
T
CC
current limit decreases as the V
graph, the user can calculate the
V
A
IN
Minimum Output Current vs V
)
)
CC
V
−
IN
10
Higher Than 4.7V
I
(V)
Q
1MHz
=
300kHz
) of the DD package can
CC
1 28
G
.
V
I
vs V
IN
DRIVE
CC
W
IN
3758 F02
voltage will fall
−
at Different
100
1 6 6 mA
current limit
CC
.
Minimum
3758f
IN
IN