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LM2734YMK Scheda tecnica(PDF) 11 Page - Texas Instruments

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Il numero della parte LM2734YMK
Spiegazioni elettronici  LM2734 Thin SOT 1A Load Step-Down DC-DC Regulator
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Produttore elettronici  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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LM2734YMK Scheda tecnica(HTML) 11 Page - Texas Instruments

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LM2734
VIN
BOOST
SW
GND
L
D1
D2
D3
R3
C4
VBOOST
CBOOST
VZ
V
IN
C
IN
V
OUT
C
OUT
LM2734
VIN
BOOST
SW
GND
CBOOST
L
D1
D2
D3
CIN
VIN
COUT
VOUT
VBOOST
LM2734
www.ti.com
SNVS288I – SEPTEMBER 2004 – REVISED APRIIL 2013
Figure 25. Zener Reduces Boost Voltage from VIN
An alternative method is to place the zener diode D3 in a shunt configuration as shown in Figure 26. A small
350mW to 500mW 5.1V zener in a SOT or SOD package can be used for this purpose. A small ceramic
capacitor such as a 6.3V, 0.1µF capacitor (C4) should be placed in parallel with the zener diode. When the
internal NMOS switch turns on, a pulse of current is drawn to charge the internal NMOS gate capacitance. The
0.1 µF parallel shunt capacitor ensures that the VBOOST voltage is maintained during this time.
Resistor R3 should be chosen to provide enough RMS current to the zener diode (D3) and to the BOOST pin. A
recommended choice for the zener current (IZENER) is 1 mA. The current IBOOST into the BOOST pin supplies the
gate current of the NMOS control switch and varies typically according to the following formula for the X version:
IBOOST = 0.56 x (D + 0.54) x (VZENER – VD2) mA
(6)
IBOOST can be calculated for the Y version using the following:
IBOOST = 0.22 x (D + 0.54) x (VZENER - VD2) µA
(7)
where D is the duty cycle, VZENER and VD2 are in volts, and IBOOST is in milliamps. VZENER is the voltage applied to
the anode of the boost diode (D2), and VD2 is the average forward voltage across D2. Note that this formula for
IBOOST gives typical current. For the worst case IBOOST, increase the current by 40%. In that case, the worst case
boost current will be
IBOOST-MAX = 1.4 x IBOOST
(8)
R3 will then be given by
R3 = (VIN - VZENER) / (1.4 x IBOOST + IZENER)
(9)
For example, using the X-version let VIN = 10V, VZENER = 5V, VD2 = 0.7V, IZENER = 1mA, and duty cycle D = 50%.
Then
IBOOST = 0.56 x (0.5 + 0.54) x (5 - 0.7) mA = 2.5mA
(10)
R3 = (10V - 5V) / (1.4 x 2.5mA + 1mA) = 1.11k
(11)
Figure 26. Boost Voltage Supplied from the Shunt Zener on VIN
Copyright © 2004–2013, Texas Instruments Incorporated
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