LM3524DM datasheet(16/29 Pages) TI

16 / 29 page

LM2524D, LM3524D

SNVS766E – JUNE 2009 – REVISED MAY 2013

www.ti.com

(10)

where: L1 is in Henrys

f is switching frequency in Hz

Also, see LM1578 data sheet for graphical methods of inductor selection.

Calculating Output Filter Capacitor Co

Figure 23 shows L1's current with respect to Q1's tON and tOFF times (VA is at the collector of Q1). This curent

must flow to the load and Co. Co's current will then be the difference between IL, and Io.

Ico = IL − Io

(11)

From Figure 23 it can be seen that current will be flowing into Co for the second half of tON through the first half of

tOFF, or a time, tON/2 + tOFF/2. The current flowing for this time is ΔIL/4. The resulting ΔVc or ΔVo is described by:

(12)

For best regulation, the inductor's current cannot be allowed to fall to zero. Some minimum load current Io, and

thus inductor current, is required as shown below:

(13)

Figure 24. Inductor Current Slope in Step-Down Regulator

A complete step-down switching regulator schematic, using the LM3524D, is illustrated in Figure 25. Transistors

Q1 and Q2 have been added to boost the output to 1A. The 5V regulator of the LM3524D has been divided in

half to bias the error amplifier's non-inverting input to within its common-mode range. Since each output

transistor is on for half the period, actually 45%, they have been paralleled to allow longer possible duty cycle, up

to 90%. This makes a lower possible input voltage. The output voltage is set by:

(14)

where VNI is the voltage at the error amplifier's non-inverting input.

Submit Documentation Feedback

Product Folder Links: LM2524D LM3524D

LM3524DM Datasheet(PDF) 16 Page - Texas Instruments