BOOST

SW

GND

L

D1

D2

COUT

CBOOST

VOUT

CIN

VIN

VIN

VBOOST

LM2736

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SNVS316H – SEPTEMBER 2004 – REVISED DECEMBER 2014

8 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

8.1 Application Information

8.1.1 Boost Function

Capacitor CBOOST and diode D2 in Figure 12 are used to generate a voltage VBOOST. VBOOST - VSW is the gate

drive voltage to the internal NMOS control switch. To properly drive the internal NMOS switch during its on-time,

VBOOST needs to be at least 1.6 V greater than VSW. Although the LM2736 device will operate with this minimum

voltage, it may not have sufficient gate drive to supply large values of output current. Therefore, it is

recommended that VBOOST be greater than 2.5 V above VSW for best efficiency. VBOOST – VSW should not exceed

the maximum operating limit of 5.5 V.

5.5 V > VBOOST – VSW > 2.5 V for best performance.

Figure 12. VOUT Charges CBOOST

When the LM2736 device starts up, internal circuitry from the BOOST pin supplies a maximum of 20 mA to

CBOOST. This current charges CBOOST to a voltage sufficient to turn the switch on. The BOOST pin will continue to

source current to CBOOST until the voltage at the feedback pin is greater than 1.18 V.

There are various methods to derive VBOOST:

1. From the input voltage (VIN)

2. From the output voltage (VOUT)

3. From an external distributed voltage rail (VEXT)

4. From a shunt or series zener diode

In the Functional Block Diagram, capacitor CBOOST and diode D2 supply the gate-drive current for the NMOS

switch. Capacitor CBOOST is charged via diode D2 by VIN. During a normal switching cycle, when the internal

NMOS control switch is off (TOFF) (refer to Figure 11), VBOOST equals VIN minus the forward voltage of D2 (VFD2),

during which the current in the inductor (L) forward biases the Schottky diode D1 (VFD1). Therefore the voltage

stored across CBOOST is

VBOOST - VSW = VIN - VFD2 + VFD1

(1)

When the NMOS switch turns on (TON), the switch pin rises to

VSW = VIN – (RDSON x IL),

(2)

forcing VBOOST to rise thus reverse biasing D2. The voltage at VBOOST is then

VBOOST = 2VIN – (RDSON x IL) – VFD2 + VFD1

(3)

which is approximately

2 VIN - 0.4 V

(4)

for many applications. Thus the gate-drive voltage of the NMOS switch is approximately

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