LM3477AMM datasheet(13/23 Pages) NSC

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Functional Description (Continued)

calculations for minimum inductance and necessary slope

resistance R

SL are carried out based on this resonant peak-

ing.

Start-up/Soft-start

The LM3477/A incorporates an internal soft-start during

start-up. The soft-start forces the inductor current to rise

slowly and smoothly as it increases towards the steady-state

current. This technique is used to reduce the input inrush

current during soft-start. The soft-start functionality is effec-

tive for approximately the first 5ms of start-up.

Note: The LM3477/A will not start-up if the output voltage is being held up

externally by more than 200mV.

If the slope resistor RSL is used, the LM3477/A may need up to 100 mA

of pre load to successfully start up.

Short Circuit Protection

When the voltage across the sense resistor (measured as

the V

IN −ISEN differential voltage) exceeds VSC, short-circuit

current limit gets activated. In the short-circuit protection

mode, the external MOSFET is turned off. When the short is

removed, the external MOSFET is turned on after five

cycles. The short circuit protection voltage V

SC is specified in

the ELECTRICAL CHARACTERISTICS section. V

lower in the LM3477A than in the LM3477.

Shutdown

The compensation pin (Pin 2) of LM3477/A also functions as

a shutdown pin. If a low signal (refer to the ELECTRICAL

CHARACTERISTICS for definition of low signal) appears on

the COMP/SD pin, the LM3477/A stops switching and goes

into a low supply current mode. The total supply current of

the IC reduces to less than 10µA under these conditions.

Figure 5 shows different implementations of the shutdown

function.

Design Section

General

Power supply design involves making tradeoffs. To achieve

performance specifications, limitations will be set on compo-

nent selection. The LM3477/A provides many degrees of

flexibility in choosing external components to accommodate

various performance/component selection optimizations. For

example, the internal slope compensation can be externally

increased to allow smaller inductances to be used. The

design procedures that follow provide instruction on how to

select the external components in a typical LM3477/A buck

circuit in continuous conduction mode, as well as aid in the

optimization of performance and/or component selection.

See Figure 6 for component reference and typical circuit.

The LM3477/A may also be designed to operate in discon-

tinuous conduction mode.

Programming the Output Voltage

The output voltage can be programmed using a resistor

divider between the output and the feedback pins, as shown

in Figure 6. The resistors are selected such that the voltage

at the feedback pin is 1.27V. R

FB1 and RFB2 can be selected

using the equation:

OUT = 1.27*(1+ RFB1/RFB2)

Calculating the Duty Cycle

In buck converter applications, the duty cycle of the

LM3477/A may be calculated as:

Where

D = forward drop of the power diode

) 0.5V

of the MOSFET when it is conducting )

OUT*RDSON

SN = Voltage across the sense resistor = IOUT xRSN

20003325

20003326

FIGURE 5. Implementing Shutdown in LM3477

200033M8

FIGURE 6. LM3477 Buck Converter Reference

Schematic

LM3477AMM Datasheet(PDF) 13 Page - National Semiconductor (TI)

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