LT3758 LINER [Linear Technology], LT3758 Datasheet - Page 13

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

APPLICATIONS INFORMATION
As illustrated in Figure 2, a trade-off between the operating
frequency and the size of the power MOSFET may be needed
in order to maintain a reliable IC junction temperature.
Prior to lowering the operating frequency, however, be
sure to check with power MOSFET manufacturers for their
most recent low Q
manufacturing technologies are continually improving, with
newer and better performance devices being introduced
almost yearly.
An effective approach to reduce the power consumption
of the internal LDO for gate drive is to tie the INTV
to an external voltage source high enough to turn off the
internal LDO regulator.
If the input voltage V
maximum rating of both the power MOSFET gate-source
voltage (V
voltage (17.5V), the INTV
to the V
turned off and the gate driver will be powered directly
from the input voltage V
V
INTV
the lowest shutdown mode input supply current, do not
connect the INTV
In SEPIC or fl yback applications, the INTV
connected to the output voltage V
diode, as shown in Figure 3, if V
conditions:
A resistor R
limit the inrush current from V
IN
1. V
2. V
3. V
, however, a small current (around 16μA) will load the
CC
OUT
OUT
OUT
in shutdown mode. For applications that require
IN
GS
pin. In this condition, the internal LDO will be
< V
< 17.5V
< maximum V
VCC
) and the INTV
IN
can be connected, as shown in Figure 3, to
(pin voltage)
CC
G
, low R
pin to V
IN
IN
GS
CC
. With the INTV
does not exceed the absolute
DS(ON)
CC
overvoltage lockout threshold
IN
rating of power MOSFET
pin can be shorted directly
.
OUT
devices. Power MOSFET
OUT
. Regardless of whether
OUT
meets the following
through a blocking
CC
pin shorted to
CC
Figure 3. Connecting INTV
LT3758
pin can be
INTV
GND
CC
CC
pin
3758 F03
C
4.7μF
VCC
D
or not the INTV
source, it is always necessary to have the driver circuitry
bypassed with a 4.7μF low ESR ceramic capacitor to ground
immediately adjacent to the INTV
Operating Frequency and Synchronization
The choice of operating frequency may be determined
by on-chip power dissipation, otherwise it is a trade-off
between effi ciency and component size. Low frequency
operation improves effi ciency by reducing gate drive cur-
rent and MOSFET and diode switching losses. However,
lower frequency operation requires a physically larger
inductor. Switching frequency also has implications for
loop compensation. The LT3758 uses a constant-frequency
architecture that can be programmed over a 100kHz to
1000kHz range with a single external resistor from the
RT pin to ground, as shown in Figure 1. The RT pin must
have an external resistor to GND for proper operation of
the LT3758. A table for selecting the value of R
operating frequency is shown in Table 1.
Table 1. Timing Resistor (R
VCC
SWITCHING FREQUENCY (kHz)
R
CC
VCC
to V
V
OUT
OUT
1000
CC
100
200
300
400
500
600
700
800
900
pin is connected to an external voltage
T
) Value
CC
and GND pins.
R
LT3758
T
63.4
41.2
30.9
24.3
19.6
16.5
12.1
10.5
140
14
(kΩ)
T
for a given
13
3758f