LM3485 datasheet(12/23 Pages) TI

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LM3485

SNVS178G – JANUARY 2002 – REVISED FEBRUARY 2013

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One problem with selecting a higher current limit is inrush current during start-up. Increasing the capacitance

(CADJ) in parallel with RADJ results in soft-start. CADJ and RADJ create an RC time constant forcing current limit to

activate at a lower current. The output voltage will ramp more slowly when using the soft-start functionality. There

are example start-up plots for CADJ equal to 1nF and 10nF in the Typical Performance Characteristics. Lower

values for CADJ will have little to no effect on soft-start.

EXTERNAL SENSE RESISTOR

The VDS of a PFET will tend to vary significantly over temperature. This will result an equivalent variation in

current limit. To improve current limit accuracy an external sense resistor can be connected from VIN to the

source of the PFET, as shown in Figure 6.

Figure 6. Current Sensing by External Resistor

PGATE

When switching, the PGATE pin swings from VIN (off) to some voltage below VIN (on). How far the PGATE will

swing depends on several factors including the capacitance, on time, and input voltage.

As shown in the Typical Performance Characteristics, PGATE voltage swing will increase with decreasing gate

capacitance. Although PGATE voltage will typically be around VIN-5V, with every small gate capacitances, this

value can increase to a typical maximum of VIN-8.3V.

Additionally, PGATE swing voltage will increase as on time increases. During long on times, such as when

operating at 100% duty cycle, the PGATE voltage will eventually fall to its maximum voltage of VIN-8.3V (typical)

regardless of the PFET gate capacitance.

The PGATE voltage will not fall below 0.4V (typical). Therefore, when the input voltage falls below approximately

9V, the PGATE swing voltage range will be reduced. At an input voltage of 7V, for instance, PGATE will swing

from 7V to a minimum of 0.4V.

DESIGN INFORMATION

Hysteretic control is a simple control scheme. However the operating frequency and other performance

characteristics highly depend on external conditions and components. If either the inductance, output

capacitance, ESR, VIN, or Cff is changed, there will be a change in the operating frequency and output ripple.

The best approach is to determine what operating frequency is desirable in the application and then begin with

the selection of the inductor and COUT ESR.

INDUCTOR SELECTION (L1)

The important parameters for the inductor are the inductance and the current rating. The LM3485 operates over

a wide frequency range and can use a wide range of inductance values. A good rule of thumb is to use the

equations used for Simple Switchers®. The equation for inductor ripple (

Δi) as a function of output current (IOUT)

is:

for Iout < 2.0Amps

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LM3485 Datasheet(PDF) 12 Page - Texas Instruments