LT3757 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT3757

Boost, Flyback, SEPIC and Inverting Controller

Linear Technology 

LT3757/LT3757A

Applications Information

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) and the INTVCC overvoltage lockout threshold

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.

LT3757

INTVCC

GND

DVCC

RVCC

VOUT

CVCC

4.7µF

3757 F03

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 LT3757 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 LT3757. A table for selecting the value of RT for a given

operating frequency is shown in Table 1.

Table 1. Timing Resistor (RT) Value

OSCILLATOR FREQUENCY (kHz)

100

200

300

400

500

600

700

800

900

1000

RT (kΩ)

140

63.4

41.2

30.9

24.3

19.6

16.5

14

12.1

10.5

The operating frequency of the LT3757 can be synchronized

to an external clock source. By providing a digital clock

signal into the SYNC pin, the LT3757 will operate at the

SYNC clock frequency. If this feature is used, an RT resistor

should be chosen to program a switching frequency 20%

slower than SYNC pulse frequency. The SYNC pulse should

have a minimum pulse width of 200ns. Tie the SYNC pin

to GND if this feature is not used.

Figure 3. Connecting INTVCC to VOUT

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