LM3533 datasheet(55/63 Pages) TI1

55 / 63 page

kHz

500

CIN

LM3533

VIN Supply

I SUPPLY

? IL

CIN

ISUPPLY

? IL

4 x

L x

RESONANT

L x

SUPPLYRIPP

kHz

500

kHz

500

LM3533

www.ti.com

SNOSC68C – APRIL 2012 – REVISED SEPTEMBER 2015

Layout Guidelines (continued)

Figure 54. Input RLC Network

10.1.2 Charge Pump

The charge pump basically has three areas of concern regarding component placement:

1. The flying capacitor (CP)

2. The output capacitor (CPOUT)

3. The input capacitor

10.1.2.1 Flying Capacitor (CP)

The charge pump flying capacitor must quickly charge up to the input voltage and then supply the current to the

output every switching cycle. Because the charge pump switching frequency is 1 MHz, the capacitor must be a

low-inductance and low-resistive ceramic. Additionally, there must be a low-inductive connection from CP to the

LM3533 flying capacitor terminals C+ and C

−. This is accomplished by placing CP as close as possible to the

LM3533 and on the same layer to avoid vias.

10.1.2.2 Output Capacitor (CPOUT)

The charge pump output capacitor sees the switched charge from the flying capacitor every switching cycle (1

MHz). This fast switching action requires that a low inductive and low resistive capacitor (ceramic) be used and

that CPOUT be connected to the LM3533 CPOUT pin with a low inductive connection. This is done by placing

CPOUT as close as possible to the CPOUT and GND pins of the LM3533 and on the same layer as the LM3533

to avoid vias.

10.1.2.3 Charge Pump Input Capacitor Placement

The input capacitor for the LM3533 charge pump is the same one used for the LM3533 inductive boost converter

(see Boost Input Capacitor Selection and Placement).

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