LT3758 datasheet(13/36 Pages) LINER

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LT3758

3758f

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 QG, low RDS(ON) devices. Power MOSFET

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 INTVCC pin

to an external voltage source high enough to turn off the

internal LDO regulator.

If the input voltage VIN does not exceed the absolute

maximum rating of both the power MOSFET gate-source

voltage (VGS)andtheINTVCCovervoltagelockoutthreshold

voltage (17.5V), the INTVCC pin can be shorted directly

to the VIN pin. In this condition, the internal LDO will be

turned off and the gate driver will be powered directly

from the input voltage VIN. With the INTVCC pin shorted to

VIN, however, a small current (around 16μA) will load the

INTVCC in shutdown mode. For applications that require

the lowest shutdown mode input supply current, do not

connect the INTVCC pin to VIN.

In SEPIC or ﬂyback applications, the INTVCC pin can be

connected to the output voltage VOUT through a blocking

diode, as shown in Figure 3, if VOUT meets the following

conditions:

1. VOUT < VIN (pin voltage)

2. VOUT < 17.5V

3. VOUT < maximum VGS rating of power MOSFET

A resistor RVCC can be connected, as shown in Figure 3, to

limit the inrush current from VOUT. Regardless of whether

or not the INTVCC pin is connected to an external voltage

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 INTVCC and GND pins.

Operating Frequency and Synchronization

The choice of operating frequency may be determined

by on-chip power dissipation, otherwise it is a trade-off

between efﬁciency and component size. Low frequency

operation improves efﬁciency 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 RT for a given

operating frequency is shown in Table 1.

Table 1. Timing Resistor (RT) Value

SWITCHING FREQUENCY (kHz)

RT (kΩ)

100

140

200

63.4

300

41.2

400

30.9

500

24.3

600

19.6

700

16.5

800

900

12.1

1000

10.5

Figure 3. Connecting INTVCC to VOUT

CVCC

4.7μF

VOUT

3758 F03

INTVCC

GND

LT3758

RVCC

DVCC

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