LM27341_14 datasheet(11/42 Pages) TI1

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VIN

-VD1

tON

D = tON/TSW

VSW

tOFF

TSW

'iL

IOUT

ILPK

LM27341, LM27342, LM27341Q, LM27342Q

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SNVS497D – NOVEMBER 2008 – REVISED FEBRUARY 2010

Figure 32. LM27341/LM27342 Waveforms of SW Pin Voltage and Inductor Current

BOOST FUNCTION

Capacitor C2 in Figure 31, commonly referred to as CBOOST, is used to store a voltage VBOOST. When the

LM27341/LM27342 starts up, an internal LDO charges CBOOST ,via an internal diode, to a voltage sufficient to

turn the internal NMOS switch on. The gate drive voltage supplied to the internal NMOS switch is VBOOST - VSW.

During a normal switching cycle, when the internal NMOS control switch is off (tOFF) (refer to Figure 32), VBOOST

equals VLDO minus the forward voltage of the internal diode (VD2). At the same time the inductor current (iL)

forward biases the catch diode D1 forcing the SW pin to swing below ground by the forward voltage drop of the

catch diode (VD1). Therefore, the voltage stored across CBOOST is

VBOOST - VSW = VLDO - VD2 + VD1

(1)

Thus,

VBOOST = VSW + VLDO - VD2 + VD1

(2)

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

VSW = VIN – (RDSON x IL),

(3)

reverse biasing D1, and forcing VBOOST to rise. The voltage at VBOOST is then

VBOOST = VIN – (RDSON x IL) + VLDO – VD2 + VD1

(4)

which is approximately

VIN + VLDO- 0.4V

(5)

VBOOST has pulled itself up by its "bootstraps", or boosted to a higher voltage.

LOW INPUT VOLTAGE CONSIDERATIONS

When the input voltage is below 5V and the duty cycle is greater than 75 percent, the gate drive voltage

developed across CBOOST might not be sufficient for proper operation of the NMOS switch. In this case, CBOOST

should be charged via an external Schottky diode attached to a 5V voltage rail, see Figure 33. This ensures that

the gate drive voltage is high enough for proper operation of the NMOS switch in the triode region. Maintain

VBOOST - VSW less than the 6V absolute maximum rating.

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