LM2736_15 Datasheet, PDF (11/36 Pages) Texas Instruments – LM2736 Thin SOT 750 mA Load Step-Down DC-DC Regulator

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8 Application and Implementation
LM2736
SNVS316H – SEPTEMBER 2004 – REVISED DECEMBER 2014
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
8.1.1 Boost Function
Capacitor C_BOOST and diode D2 in Figure 12 are used to generate a voltage V_BOOST. V_BOOST - V_SW is the gate
drive voltage to the internal NMOS control switch. To properly drive the internal NMOS switch during its on-time,
V_BOOST needs to be at least 1.6 V greater than V_SW. Although the LM2736 device 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 V_BOOST be greater than 2.5 V above V_SW for best efficiency. V_BOOST – V_SW should not exceed
the maximum operating limit of 5.5 V.
5.5 V > V_BOOST – V_SW > 2.5 V for best performance.
V_IN
C_IN
V_BOOST
D2
V_IN
BOOST
SW
GND
C_BOOST
L
D1
V_OUT
C_OUT
Figure 12. V_OUT Charges C_BOOST
When the LM2736 device starts up, internal circuitry from the BOOST pin supplies a maximum of 20 mA to
C_BOOST. This current charges C_BOOST to a voltage sufficient to turn the switch on. The BOOST pin will continue to
source current to C_BOOST until the voltage at the feedback pin is greater than 1.18 V.
There are various methods to derive V_BOOST:
1. From the input voltage (V_IN)
2. From the output voltage (V_OUT)
3. From an external distributed voltage rail (V_EXT)
4. From a shunt or series zener diode
In the Functional Block Diagram, capacitor C_BOOST and diode D2 supply the gate-drive current for the NMOS
switch. Capacitor C_BOOST is charged via diode D2 by V_IN. During a normal switching cycle, when the internal
NMOS control switch is off (T_OFF) (refer to Figure 11), V_BOOST equals V_IN minus the forward voltage of D2 (V_FD2),
during which the current in the inductor (L) forward biases the Schottky diode D1 (V_FD1). Therefore the voltage
stored across C_BOOST is
V_BOOST - V_SW = V_IN - V_FD2 + V_FD1
(1)
When the NMOS switch turns on (T_ON), the switch pin rises to
V_SW = V_IN – (R_DSON x I_L),
(2)
forcing V_BOOST to rise thus reverse biasing D2. The voltage at V_BOOST is then
V_BOOST = 2V_IN – (R_DSON x I_L) – V_FD2 + V_FD1
(3)
which is approximately
2 V_IN - 0.4 V
(4)
for many applications. Thus the gate-drive voltage of the NMOS switch is approximately
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