LM3532 datasheet(44/52 Pages) TI2

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LM3532

SNVS653E – JULY 2011 – REVISED AUGUST 2015

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Layout Guidelines (continued)

10.1.4 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 milliamps with 5 ns rise and fall times. This appears 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 because 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 is underdamped and

has 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).

Equation 1 is the criteria for an underdamped response. Equation 2 is the resonant frequency. Equation 3 is the

approximated supply current ripple as a function of LS, RS, and CIN.

As an example, consider a 3.6-V supply with 0.1

Ω of series resistance connected to CIN through 50 nH of

connecting traces. This results in an underdamped input filter circuit with a resonant frequency of 712 kHz.

Because the switching frequency lies near to the resonant frequency of the input RLC network, the supply

current is probably larger then the inductor current ripple. In this case, using equation 3 from Figure 45, the

supply current ripple can be approximated as 1.68 times the inductor current ripple. Increasing the series

inductance (LS) to 500 nH causes the resonant frequency to move to around 225 kHz and the supply current

ripple to be approximately 0.25 times the inductor current ripple.

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