LM3532 datasheet(36/52 Pages) TI2

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IOUT x VOUT

VIN x efficiency

IPEAK =

VIN

2 x fsw x L

1 -

VOUT

VIN x efficiency

LM3532

SNVS653E – JULY 2011 – REVISED AUGUST 2015

www.ti.com

8.2.2.2 Capacitor Selection

The LM3532’s output capacitor has two functions: filtering of the boost converter's switching ripple, and to ensure

feedback loop stability. As a filter, the output capacitor supplies the LED current during the boost converter's on

time and absorbs the inductor's energy during the switch's off time. This causes a sag in the output voltage

during the on time and a rise in the output voltage during the off time. Because of this, the output capacitor must

be sized large enough to filter the inductor current ripple that could cause the output voltage ripple to become

excessive. As a feedback loop component, the output capacitor must be at least 1µF and have low ESR;

otherwise, the LM3532's boost converter can become unstable. This requires the use of ceramic output

capacitors. Table 23 lists part numbers and voltage ratings for different output capacitors that can be used with

the LM3532.

Table 23. Input/Output Capacitors

MANUFACTURER

PART NUMBER

VALUE

SIZE

RATING

DESCRIPTION

Murata

GRM21BR71H105KA12

1 µF

0805

50 V

COUT

Murata

GRM188B31A225KE33

2.2 µF

0805

10 V

CIN

TDK

C1608X5R0J225

2.2 µF

0603

6.3 V

CIN

8.2.2.3 Diode Selection

The diode connected between the SW and OUT pins must be a Schottky diode and have a reverse breakdown

voltage high enough to handle the maximum output voltage in the application. Table 24 lists various diodes that

can be used with the LM3532.

Table 24. Diodes

MANUFACTURER

PART NUMBER

VALUE

SIZE

RATING

Diodes Inc.

B0540WS

Schottky

SOD-323

40/500 V/mA

Diodes Inc.

SDM20U40

Schottky

SOD-523 (1.2 mm × 0.8 mm ×

40/200 V/mA

0.6 mm)

On Semiconductor

NSR0340V2T1G

Schottky

SOD-523 (1.2 mm × 0.8 mm ×

40/250 V/mA

0.6 mm)

On Semiconductor

NSR0240V2T1G

Schottky

SOD-523 (1.2 mm × 0.8 mm ×

40/250 V/mA

0.6 mm)

8.2.2.4 Maximum Output Power

The LM3532 device's maximum output power is governed by two factors: the peak current limit (ICL = 880 mA

minimum), and the maximum output voltage (VOVP = 40 V minimum). When the application causes either of these

limits to be reached it is possible that the proper current regulation and matching between LED current strings

may not be met.

8.2.2.4.1

Peak Current Limited

In the case of a peak current limited situation, when the peak of the inductor current hits the LM3532's current

limit the NFET switch turns off for the remainder of the switching period. If this happens, each switching cycle the

LM3532 begins to regulate the peak of the inductor current instead of the headroom across the current sinks.

This can result in the dropout of the feedback-enabled current sinks and the current dropping below its

programmed level.

The peak current in a boost converter is dependent on the value of the inductor, total LED current (IOUT), the

output voltage (VOUT) (which is the highest voltage LED string + 0.4 V regulated headroom voltage), the input

voltage VIN, and the efficiency (output power/input power). Additionally, the peak current is different depending on

whether the inductor current is continuous during the entire switching period (CCM) or discontinuous (DCM)

where it goes to 0 before the switching period ends.

For CCM the peak inductor current is given by:

(5)

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