LM3532TMX-40ASLASHNOPB datasheet(39/47 Pages) TI1

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LM3532

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SNVS653D – JULY 2011 – REVISED JUNE 2013

Output Capacitor Placement

The output capacitor is in the path of the inductor current discharge current. As a result, COUT sees a high current

step from 0 to IPEAK each time the switch turns off and the Schottky diode turns on. Typical turn-off/turn-on times

are around 5ns. Any inductance along this series path from the cathode of the diode through COUT and back into

the LM3532's GND pin will contribute to voltage spikes (VSPIKE = LPX × dI/dt) at SW and OUT which can

potentially over-voltage the SW pin, or feed through to GND. To avoid this, COUT+ must be connected as close as

possible to the Cathode of the Schottky diode and COUT− must be connected as close as possible to the

LM3532's GND bump. The best placement for COUT is on the same layer as the LM3532 so as to avoid any vias

that will add extra series inductance (see Example Layouts).

Schottky Diode Placement

The Schottky diode is in the path of the inductor current discharge. As a result the Schottky diode sees a high

current step from 0 to IPEAK each time the switch turns off and the diode turns on. Any inductance in series with

the diode will cause a voltage spike (VSPIKE = LPX × dI/dt) at SW and OUT which can potentially over-voltage the

SW pin, or feed through to VOUT and through the output capacitor and into GND. Connecting the anode of the

diode as close as possible to the SW pin and the cathode of the diode as close as possible to COUT+ will

reduce the inductance (LPX) and minimize these voltage spikes (See Example Layouts).

Inductor Placement

The node where the inductor connects to the LM3532’s SW bump has 2 issues. First, a large switched voltage (0

to VOUT + VF_SCHOTTKY) appears on this node every switching cycle. This switched voltage can be capacitively

coupled into nearby nodes. Second, there is a relatively large current (input current) on the traces connecting the

input supply to the inductor and connecting the inductor to the SW bump. Any resistance in this path can cause

large voltage drops that will negatively affect efficiency.

To reduce the capacitively coupled signal from SW into nearby traces, the SW bump to inductor connection must

be minimized in area. This limits the PCB capacitance from SW to other traces. Additionally, other nodes need to

be routed away from SW and not directly beneath. This is especially true for high impedance nodes that are

more susceptible to capacitive coupling such as (SCL, SDA, HWEN, PWM, and possibly ASL1 and ALS2). A

GND plane placed directly below SW will help isolate SW and dramatically reduce the capacitance from SW into

nearby traces.

To limit the trace resistance of the VBATT to inductor connection and from the inductor to SW connection, use

short, wide traces (see Example Layouts).

Input Capacitor Selection and Placement

The input bypass capacitor filters the inductor current ripple, and the internal MOSFET driver currents during turn

on of the power switch.

The driver current requirement can be a few hundred mA's with 5ns rise and fall times. This will appear as high

dI/dt current pulses coming from the input capacitor each time the switch turns on. Close placement of the input

capacitor to the IN pin and to the GND pin is critical since any series inductance between IN and CIN+ or CIN−

and GND can create voltage spikes that could appear on the VIN supply line and in the GND plane.

Close placement of the input bypass capacitor at the input side of the inductor is also critical. The source

impedance (inductance and resistance) from the input supply, along with the input capacitor of the LM3532, form

a series RLC circuit. If the output resistance from the source (RS) is low enough the circuit will be underdamped

and will have a resonant frequency (typically the case). Depending on the size of LS the resonant frequency

could occur below, close to, or above the LM3532's switching frequency. This can cause the supply current ripple

to be:

•

Approximately equal to the inductor current ripple when the resonant frequency occurs well above the

LM3532's switching frequency;

•

Greater then the inductor current ripple when the resonant frequency occurs near the switching frequency; or

•

Less then the inductor current ripple when the resonant frequency occurs well below the switching frequency.

Figure 45 shows this series RLC circuit formed from the output impedance of the supply and the input capacitor.

The circuit is re-drawn for the AC case where the VIN supply is replaced with a short to GND and the LM3532 +

Inductor is replaced with a current source (

ΔIL).

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