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LM3485

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SNVS178F – MAY 2004 – REVISED SEPTEMBER 2009

EXTERNAL SENSE RESISTOR

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

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

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

Δi) as a function of output

−0.366726

The inductance can be calculated based upon the desired operating frequency where:

(7)

And

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