LM2736 datasheet(8/22 Pages) NSC

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Application Information (Continued)

current. Therefore, it is recommended that V

BOOST

greater than 2.5V above V

SW for best efficiency. VBOOST –

SW should not exceed the maximum operating limit of 5.5V.

5.5V > V

BOOST –VSW > 2.5V for best performance.

When the LM2736 starts up, internal circuitry from the

BOOST pin supplies a maximum of 20mA 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

BOOST until the voltage at the feedback pin is greater than

1.18V.

There are various methods to derive V

BOOST:

From the input voltage (V

IN)

From the output voltage (V

OUT)

From an external distributed voltage rail (V

EXT)

From a shunt or series zener diode

In the Simplifed Block Diagram of Figure 1, capacitor

BOOST and diode D2 supply the gate-drive current for the

NMOS switch. Capacitor C

BOOST is charged via diode D2 by

IN. During a normal switching cycle, when the internal

NMOS control switch is off (T

OFF) (refer to Figure 2), VBOOST

equals V

IN minus the forward voltage of D2 (VFD2), during

which the current in the inductor (L) forward biases the

Schottky diode D1 (V

FD1). Therefore

the voltage stored

across C

BOOST is

BOOST -VSW =VIN -VFD2 +VFD1

When the NMOS switch turns on (T

ON), the switch pin rises

SW =VIN –(RDSON xIL),

forcing V

BOOST to rise thus reverse biasing D2. The voltage

at V

BOOST is then

BOOST =2VIN –(RDSON xIL)– VFD2 +VFD1

which is approximately

IN - 0.4V

for many applications. Thus the gate-drive voltage of the

NMOS switch is approximately

IN - 0.2V

An alternate method for charging C

BOOST is to connect D2 to

the output as shown in Figure 3. The output voltage should

be between 2.5V and 5.5V, so that proper gate voltage will

be applied to the internal switch. In this circuit, C

BOOST

provides a gate drive voltage that is slightly less than V

OUT.

In applications where both V

IN and VOUT are greater than

5.5V, or less than 3V, C

BOOST cannot be charged directly

from these voltages. If V

IN and VOUT are greater than 5.5V,

BOOST can be charged from VIN or VOUT minus a zener

voltage by placing a zener diode D3 in series with D2, as

shown in Figure 4. When using a series zener diode from the

input, ensure that the regulation of the input supply doesn’t

create a voltage that falls outside the recommended V

BOOST

voltage.

INMAX –VD3) < 5.5V

INMIN –VD3) > 1.6V

An alternative method is to place the zener diode D3 in a

shunt configuration as shown in Figure 5. A small 350mW to

500mW 5.1V zener in a SOT-23 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 V

BOOST voltage is maintained during this time.

Resistor R3 should be chosen to provide enough RMS cur-

rent to the zener diode (D3) and to the BOOST pin. A

recommended choice for the zener current (I

ZENER)is1mA.

The current I

BOOST into the BOOST pin supplies the gate

current of the NMOS control switch and varies typically

according to the following formula:

BOOST = 0.56 x (D + 0.54) x (VZENER –VD2)mA

where D is the duty cycle, V

ZENER and VD2 are in volts, and

BOOST is in milliamps. VZENER is the voltage applied to the

anode of the boost diode (D2), and V

D2 is the average

forward voltage across D2. Note that this formula for I

BOOST

gives typical current. For the worst case I

BOOST, increase the

current by 40%. In that case, the worst case boost current

will be

BOOST-MAX =1.4xIBOOST

R3 will then be given by

R3=(V

IN -VZENER)/(1.4xIBOOST +IZENER)

For example, let V

IN = 10V, VZENER =5V, VD2 = 0.7V, IZENER

= 1mA, and duty cycle D = 50%. Then

BOOST = 0.56 x (0.5 + 0.54) x (5 - 0.7) mA = 2.5mA

R3 = (10V - 5V) / (1.4 x 2.5mA + 1mA) = 1.11k

Ω

20124208

FIGURE 3. V

OUT Charges CBOOST

20124209

FIGURE 4. Zener Reduces Boost Voltage from V

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