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BOOST

GND

COUT

CBOOST

VOUT

CIN

VIN

VBOOST

VIN

TON

Inductor

Current

D = TON/TSW

VSW

TOFF

TSW

IPK

Voltage

LM2736

SNVS316F – SEPTEMBER 2004 – REVISED APRIL 2013

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Figure 20. LM2736 Waveforms of SW Pin Voltage and Inductor Current

BOOST FUNCTION

Capacitor CBOOST and diode D2 in Figure 21 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 1.6V greater than VSW. Although the LM2736 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 2.5V above VSW for best efficiency. VBOOST – VSW should not exceed the maximum

operating limit of 5.5V.

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

Figure 21. VOUT Charges CBOOST

When the LM2736 starts up, internal circuitry from the BOOST pin supplies a maximum of 20mA 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 1.18V.

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 Simplifed Block Diagram of Figure 19, 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 20), 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)

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