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LM27341 데이터시트(PDF) 11 Page - Texas Instruments

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부품명 LM27341
상세설명  2 MHz 1.5A/2A Wide Input Range Step-Down DC-DC Regulator with Frequency Synchronization
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0
0
VIN
-VD1
tON
t
t
D = tON/TSW
VSW
tOFF
TSW
iL
'iL
IOUT
ILPK
LM27341, LM27342, LM27341Q, LM27342Q
www.ti.com
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.
Copyright © 2008–2010, Texas Instruments Incorporated
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Product Folder Links: LM27341 LM27342 LM27341Q LM27342Q


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