LM3530 datasheet(40/50 Pages) TI1

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LM3530

SNVS606L – JUNE 2009 – REVISED DECEMBER 2014

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Layout Guidelines (continued)

The driver current requirement can range from 50 mA at 2.7 V to over 200 mA at 5.5 V with fast durations of

approximately 10 ns to 20 ns. 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 LM3530, 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 switching frequency of the device. This can cause the supply current ripple

to be:

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

LM3530 switching frequency;

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

and

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

Figure 57 shows the 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 LM3530 +

Inductor is replaced with a current source (

ΔIL). In Figure 57 below,

1. = the criteria for an underdamped response.

2. = the resonant frequency, and

3. = 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. Since

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 2 from Figure 57 the supply current

ripple can be approximated as 1.68 multiplied by the inductor current ripple. Increasing the series inductance (LS)

to 500 nH causes the resonant frequency to move to around 225 kHz and the supple current ripple to be

approximately 0.25 multiplied by the inductor current ripple.

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