LM2738_15 datasheet(11/38 Pages) TI

Nome de Peças	LM2738

Descrição Electrónicos	LM2738 550kHz/1.6MHz 1.5A Step-Down DC-DC Switching Regulator

Download	38 Pages

Scroll/Zoom	100%

Fabricante Electrônico	TI [Texas Instruments]

Página de início	http://www.ti.com

Logo

11 / 38 page

LM2738

BOOST

SW

GND

L

D1

D2

COUT

CBOOST

VOUT

CIN

VIN

VBOOST

LM2738

www.ti.com

SNVS556B – APRIL 2008 – REVISED APRIL 2013

BOOST FUNCTION

Capacitor CBOOST and diode D2 in Figure 29 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 needs to be at least 2.5V greater than VSW. It is recommended that VBOOST be greater than 2.5V above

VSW for best efficiency. VBOOST – VSW should not exceed the maximum operating limit of 5.5V.

5.5V > VBOOST – VSW > 2.5V for best performance.

When the LM2738 starts up, internal circuitry from the BOOST pin supplies a maximum of 20mA to CBOOST. This

current charges CBOOST to a voltage sufficient to turn the switch on. The BOOST pin will continue to source

current to CBOOST until the voltage at the feedback pin is greater than 0.76V.

There are various methods to derive VBOOST:

1. From the input voltage (3.0V < VIN < 5.5V)

2. From the output voltage (2.5V < VOUT < 5.5V)

3. From an external distributed voltage rail (2.5V < VEXT < 5.5V)

4. From a shunt or series zener diode

In the Simplifed Block Diagram of (Figure 27), 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 28), 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

VBOOST - VSW = VIN - VFD2 + VFD1

(1)

When the NMOS switch turns on (TON), the switch pin rises to

VSW = VIN – (RDSON x IL),

(2)

forcing VBOOST to rise, thus reverse biasing D2. The voltage at VBOOST is then

VBOOST = 2VIN – (RDSON x IL) – VFD2 + VFD1

(3)

which is approximately

2VIN - 0.4V

(4)

for many applications. Thus the gate-drive voltage of the NMOS switch is approximately

VIN - 0.2V

(5)

An alternate method for charging CBOOST is to connect D2 to the output as shown in Figure 29. 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, CBOOST provides a gate drive voltage that is slightly less than VOUT.

Figure 29. VOUT Charges CBOOST

In applications where both VIN and VOUT are greater than 5.5V, or less than 3V, CBOOST cannot be charged

directly from these voltages. If VIN and VOUT are greater than 5.5V, CBOOST 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 30. 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 VBOOST voltage.

(VINMAX – VD3) < 5.5V

(VINMIN – VD3) > 2.5V

Submit Documentation Feedback

11

Product Folder Links: LM2738

Nº de peça semelhante - LM2738_15

Fabricante Electrônico	Nome de Peças	Folha de dados	Descrição Electrónicos
Texas Instruments	LM2738	1Mb / 40P	[Old version datasheet] 1.5A Step-Down DC-DC Switching Regulator

More results

Descrição semelhante - LM2738_15

Fabricante Electrônico	Nome de Peças	Folha de dados	Descrição Electrónicos
National Semiconductor ...	LM2738	605Kb / 28P	550kHz/1.6MHz 1.5A Step-Down DC-DC Switching Regulator
Texas Instruments	LM2738	1Mb / 40P	[Old version datasheet] 1.5A Step-Down DC-DC Switching Regulator
Eutech Microelectronics...	EUP3291	736Kb / 1P	1.5A, 2.25MHz Synchronous Step-Down DC/DC Regulator
Texas Instruments	LM2833	1Mb / 30P	[Old version datasheet] Step-Down DC-DC Switching Regulator
National Semiconductor ...	LM2831	890Kb / 27P	High Frequency 1.5A Load - Step-Down DC-DC Regulator
Texas Instruments	LM2831	1Mb / 34P	[Old version datasheet] High Frequency 1.5A Load - Step-Down DC-DC Regulator
STMicroelectronics	L5983	938Kb / 37P	1.5A step-down switching regulator
	L5971	96Kb / 11P	1.5A STEP DOWN SWITCHING REGULATOR
	E-L4971	574Kb / 13P	1.5A STEP DOWN SWITCHING REGULATOR
	L4971	574Kb / 13P	1.5A STEP DOWN SWITCHING REGULATOR

More results

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

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

Nº de peça semelhante - LM2738_15

Descrição semelhante - LM2738_15

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

Folha de dados Download

Ligação URL