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LM2738 Folha de dados(PDF) 11 Page - Texas Instruments

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Nome de Peças LM2738
Descrição Electrónicos  1.5A Step-Down DC-DC Switching Regulator
Download  40 Pages
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Fabricante Electrônico  TI1 [Texas Instruments]
Página de início  http://www.ti.com
Logo TI1 - Texas Instruments

LM2738 Folha de dados(HTML) 11 Page - Texas Instruments

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background image
L
R1
R2
D1
D2
BOOST
Output
Control
Logic
Current
Limit
Thermal
Shutdown
Under
Voltage
Lockout
Corrective Ramp
Reset
Pulse
PWM
Comparator
Current-Sense Amplifier
RSENSE
+
+
Internal
Regulator
and
Enable
Circuit
Oscillator
Driver
0.25:
Switch
Internal
Compensation
SW
EN
FB
GND
Error Amplifier
-
+
VREF
0.8V
COUT
ON
OFF
VBOOST
IL
VSW
+
-
CBOOST
VOUT
CIN
VIN
VIN
ISENSE
+
-
+
-
0.93V
-
+
OVP
Comparator
Error
Signal
11
LM2738
www.ti.com
SNVS556C – APRIL 2008 – REVISED JANUARY 2016
Product Folder Links: LM2738
Submit Documentation Feedback
Copyright © 2008–2016, Texas Instruments Incorporated
7.2 Functional Block Diagram
7.3 Feature Description
7.3.1 Boost Function
Capacitor CBOOST and diode D2 in Figure 26 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 must be at least 2.5 V greater than VSW. TI recommends that VBOOST be greater than 2.5 V above VSW for
best efficiency. VBOOST – VSW must not exceed the maximum operating limit of 5.5 V. For best performance, see
Equation 1.
5.5 V > VBOOST – VSW > 2.5 V
(1)
When the LM2738 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 continues to source current
to CBOOST until the voltage at the feedback pin is greater than 0.76 V.
There are various methods to derive VBOOST:
1. From the input voltage (3 V < VIN < 5.5 V)
2. From the output voltage (2.5 V < VOUT < 5.5 V)
3. From an external distributed voltage rail (2.5 V < VEXT < 5.5 V)
4. From a shunt or series Zener diode
As seen on the Functional Block Diagram, capacitor CBOOST and diode D2 supply the gate-drive voltage 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 25), 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 Equation 2:
VBOOST – VSW = VIN – VFD2 + VFD1
(2)
When the NMOS switch turns on (TON), the switch pin rises to Equation 3:
VSW = VIN – (RDSON × IL),
(3)
forcing VBOOST to rise, thus reverse biasing D2. The voltage at VBOOST is then Equation 4:
VBOOST = 2 VIN – (RDSON × IL) – VFD2 + VFD1
(4)
which is approximately 2 VIN – 0.4 V for many applications. Thus the gate-drive voltage of the NMOS switch is
approximately VIN – 0.2 V.


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