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LT3740

3740fc

APPLICATIONS INFORMATION

Figure 1. MOSFET RDS(ON) vs. Temperature

The

ρT term is a normalization factor (unity at 25°C)

accounting for the signiﬁcant variation in on-resistance

with temperature, typically about 0.4%/°C as shown in

Figure 1. For a maximum junction temperature of 100°C,

using a value

ρT = 1.3 is reasonable.

Gate Drives

The top gate drive power is provided by BIAS which is about

7.8V higher than VIN. The top gate voltage can be as high

as 7.8V and can droop to about 5.5V if the on-time is long

enough. The bottom gate drive power is provided by the

BGDP pin. BGDP needs to be connected to 7V or higher

to get enough gate drive voltage for logic-level threshold

MOSFETs. BGDP can be connected to VIN, BIAS or an

external voltage supply. For input voltages lower than

7V, BGDP should be connected to BIAS to be able to use

logic-level threshold MOSFETs. For VIN higher than 7V,

BGDP can be connected to VIN to reduce power loss in the

bottom gate drive. For high BGDP voltages, the internal

clamp circuit limits the bottom gate drive voltage to about

8V to prevent the gate from overvoltage damage.

For the case BGDP is connected to VIN, if VIN voltage ramp

up slowly during startup, there will be a considerable period

of time that BGDP is below 7V and the circuit is operating.

The insufﬁcient voltage on BGDP could cause malfunction

TJ – JUNCTION TEMPERATURE (°C)

–50

0.4

1.2

0.8

1.6

2.0

–25

3740 F01

100 125 150

VGS = 10V

ID = 14A

of the circuit. One of the solution circuits is shown in

Figure 2. The Zener diode and the small MOSFET limit the

SHDN voltage to be about 6V below VIN. This shuts down

the LT3740 for VIN lower than 7V. If VIN can ramp up to

7V quick enough, this circuit is not necessary.

For VIN higher than 14V, the high dv/dt at SW node and

the strong drive of BGATE can generate extra noise and

affect the operation. A resistor RBG of 1Ω-2Ω between

BGATE and the gate of the bottom MOSFET as shown in

Figure 3 can effectively reduce the noise.

The LT3740 uses adaptive dead time control to prevent

the top and bottom MOSFET shoot-through and minimize

the dead time. When the internal top MOSFET on signal

comes, the LT3740 delays the turn on of TGATE until

BGATE is off. When the internal bottom MOSFET on signal

comes, the LT3740 delays the turn on of BGATE until the

SW node swings down to ground. In the case of small

or negative inductor current that SW node cannot swing

below ground after TGATE turns off, BGATE will turn on

200ns after TGATE is off.

Figure 2. Circuit That Prevents Operation for VIN < 7V

Figure 3. Noise Reduction for Bottom MOSFET

VIN

SHDN

BGDP

LT3740

2N7002TA

100k

MMSZ52312BS

3740 F02

PGND

BGATE

LT3740

RBG

3740 F03

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