LP3972 datasheet(49/59 Pages) TI1

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0.5

1.0

1.5

LOAD CURRENT (A)

3.0

3.5

4.0

4.5

5.0

5.5

VPP-RMS =

VPP-C

2 + V

PP-ESR

LP3972

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SNVS468J – SEPTEMBER 2006 – REVISED AUGUST 2011

Voltage peak-to-peak ripple due to ESR can be expressed as follows

VPP-ESR = (2 * IRIPPLE) * RESR

(5)

Because these two components are out of phase the RMS value can be used to get an approximate value of

peak-to-peak ripple.

Voltage peak-to-peak ripple, root mean squared can be expressed as follows

(6)

Note that the output voltage ripple is dependent on the inductor current ripple and the equivalent series

resistance of the output capacitor (RESR).

The RESR is frequency dependent (as well as temperature dependent); make sure the value used for calculations

is at the switching frequency of the part.

Table 2. Suggested Capacitor and Their Suppliers

Case Size

Model

Type

Vendor

Voltage

Inch (mm)

GRM21BR60J106K

Ceramic, X5R

Murata

6.3V

0805 (2012)

JMK212BJ106K

Ceramic, X5R

Taiyo-Yuden

6.3V

0805 (2012)

C2012X5R0J106K

Ceramic, X5R

TDK

6.3V

0805 (2012)

Buck Output Ripple Management

If VIN and ILOAD increase, the output ripple associated with the Buck Regulators also increases. Figure 32 shows

the safe operating area. To ensure operation in the area of concern it is recommended that the system designer

circumvents the output ripple issues to install Schottky diodes on the Buck(s) that are expected to perform under

these extreme corner conditions.

(Schottky diodes are recommended to reduce the output ripple, if system requirements include this shaded area

of operation. VIN > 1.5V and ILOAD > 1.24)

Figure 32.

Board Layout Considerations

PC board layout is an important part of DC-DC converter design. Poor board layout can disrupt the performance

of a DC-DC converter and surrounding circuitry by contributing to EMI, ground bounce, and resistive voltage loss

in the traces. These can send erroneous signals to the DC-DC converter IC, resulting in poor regulation or

instability.

Good layout for the converters can be implemented by following a few simple design rules.

1. Place the converters, inductor and filter capacitors close together and make the traces short. The traces

between these components carry relatively high switching currents and act as antennas. Following this rule

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