LM3485MM datasheet(9/23 Pages) TI

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LM3485

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SNVS178G – JANUARY 2002 – REVISED FEBRUARY 2013

The LM3485 operates in discontinuous conduction mode at light load current or continuous conduction mode at

heavy load current. In discontinuous conduction mode, current through the inductor starts at zero and ramps up

to the peak, then ramps down to zero. Next cycle starts when the FB voltage reaches the internal voltage. Until

then, the inductor current remains zero. Operating frequency is lower and switching losses reduce. In continuous

conduction mode, current always flows through the inductor and never ramps down to zero.

The output voltage (VOUT) can be programmed by 2 external resistors. It can be calculated as follows:

VOUT = 1.242* ( R1 + R2 ) / R2

(1)

Figure 2. Hysteretic Window

The minimum output voltage ripple (VOUT_PP) can be calculated in the same way.

VOUT_PP = VHYST ( R1 + R2 ) / R2

(2)

For example, with VOUT set to 3.3V, VOUT_PP is 26.6mV

VOUT_PP = 0.01* ( 33K + 20K ) / 20K = 0.0266V

(3)

Operating frequency (F) is determined by knowing the input voltage, output voltage, inductor, VHYST, ESR

(Equivalent Series Resistance) of output capacitor, and the delay. It can be approximately calculated using the

formula:

(4)

where:

α: ( R1 + R2 ) / R2

delay: It includes the LM3485 propagation delay time and the PFET delay time. The propagation delay is 90ns

typically (see Figure 3).

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