LM3489 datasheet(11/25 Pages) TI1

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INPUT VOLTAGE - OUTPUT VOLTAGE (V)

100

120

140

L=10PH

L=4.7PH

L=22PH

LM3489

LM3489-Q1

www.ti.com

SNVS443B – MAY 2006 – REVISED FEBRUARY 2013

Figure 25. Propagation Delay

The operating frequency and output ripple voltage can also be significantly influenced by the speed up capacitor

(Cff). Cff is connected in parallel with the high side feedback resistor, R1. The location of this capacitor is similar

to where a phase lead capacitor would be located in a PWM control scheme. However it's effect on hysteretic

operation is much different. Cff effectively shorts out R1 at the switching frequency and applies the full output

ripple to the FB pin without dividing by the R2/R1 ratio. The end result is a reduction in output ripple and an

increase in operating frequency. When adding Cff, calculate the formula above with

α = 1. The value of Cff

depend on the desired operating frequency and the value of R2. A good starting point is 470pF ceramic at

100kHz decreasing linearly with increased operating frequency. Also note that as the output voltage is

programmed below 2.5V, the effect of Cff will decrease significantly.

CURRENT LIMIT OPERATION

The LM3489 has a cycle-by-cycle current limit. Current limit is sensed across the VDS of the PFET or across an

additional sense resistor. When current limit is activated, the LM3489 turns off the external PFET for a period of

9µs(typical). The current limit is adjusted by an external resistor, RADJ.

The current limit circuit is composed of the ISENSE comparator and the one-shot pulse generator. The positive

input of the ISENSE comparator is the ADJ pin. An internal 5.5µA current sink creates a voltage across the

external RADJ resistor. This voltage is compared to the voltage across the PFET or sense resistor. The ADJ

voltage can be calculated as follows:

VADJ = VIN − (RADJ x 3.0µA)

(5)

Where 3.0µA is the minimum ICL-ADJ value.

The negative input of the ISENSE comparator is the ISENSE pin that should be connected to the drain of the

external PFET. The inductor current is determined by sensing the VDS. It can be calculated as follows.

VISENSE = VIN − (RDSON x IIND_PEAK) = VIN − VDS

(6)

Figure 26. Current Sensing by VDS

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LM3489 Datasheet(PDF) 11 Page - Texas Instruments