LM3478MASLASHNOPB.B datasheet(20/43 Pages) TI2

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ISWLIMIT =

IOUT

(1-D)

(D x VIN)

(2 x fS x L)

RSEN =

VSENSE - (D x VSL)

ISWLIMIT

0.000

0.100

0.200

0.300

0.400

0.500

CURRENT SENSE VOLTAGE (V)

100

120

FS =

250 kHz

VSENSE

VSL

FS = 500 kHz

LM3478

SNVS085X – JULY 2000 – REVISED DECEMBER 2017

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Typical Applications (continued)

8.2.1.2.3

Programming the Output Voltage

The output voltage can be programmed using a resistor divider between the output and the FB pin. The resistors

are selected such that the voltage at the FB pin is 1.26 V. Pick RF1 (the resistor between the output voltage and

the feedback pin) and RF2 (the resistor between the feedback pin and ground) can be selected using the

following equation,

RF2 = (1.26 V x RF1) / (Vout - 1.26 V)

(15)

A 100-pF capacitor may be connected between the feedback and ground pins to reduce noise.

8.2.1.2.4

Setting the Current Limit

The maximum amount of current that can be delivered to the load is set by the sense resistor, RSEN. Current limit

occurs when the voltage that is generated across the sense resistor equals the current sense threshold voltage,

VSENSE. When this threshold is reached, the switch will be turned off until the next cycle. Limits for VSENSE are

specified in the electrical characteristics section. VSENSE represents the maximum value of the internal control

signal VCS as shown in Figure 30. This control signal, however, is not a constant value and changes over the

course of a period as a result of the internal compensation ramp (VSL). Therefore the current limit threshold will

also change. The actual current limit threshold is a function of the sense voltage (VSENSE) and the internal

compensation ramp:

RSEN x ISWLIMIT = VCSMAX = VSENSE - (D x VSL)

(16)

Where ISWLIMIT is the peak switch current limit, defined by Equation 17.

Figure 30. Current Sense Voltage vs Duty Cycle

Figure 30 shows how VCS (and current limit threshold voltage) change with duty cycle. The curve is equivalent to

the internal compensation ramp slope (Se) and is bounded at low duty cycle by VSENSE, shown as a dotted line.

As duty cycle increases, the control voltage is reduced as VSL ramps up. The graph also shows the short circuit

current limit threshold of 343 mV (typical) during the 325 ns (typical) blanking time. For higher frequencies this

fixed blanking time obviously occupies more duty cycle, percentage wise. Since current limit threshold varies with

duty cycle, the use Equation 17 to select RSEN and set the desired current limit threshold:

(17)

The numerator of Equation 17 is VCS, and ISWLIMIT using Equation 18.

(18)

LM3478MA/NOPB.B Datasheet(PDF) 20 Page - Texas Instruments

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