OUTPUT CAPACITOR

The switching converter in the LP5550 is designed to be

used with a 10uF ceramic output capacitor. The dielectric

should be X5R, X7R, or comparable material to maintain

proper tolerances. The output capacitor of the switching

converter absorbs the AC ripple current from the inductor

and provides the initial response to a load transient. The

ripple voltage at the output of the converter is the product of

the ripple current flowing through the output capacitor and

the impedance of the capacitor. The impedance of the ca-

pacitor can be dominated by capacitive, resistive, or induc-

tive elements within the capacitor, depending on the fre-

quency of the ripple current. Ceramic capacitors are

predominately used in portable systems and have very low

ESR and remain capacitive up to high frequencies.

The switcher peak - to - peak output voltage ripple in steady

state can be calculated as:

LDO INFORMATION

The LDOs included in the LP5550 provide static supply

voltages for various functions in the processor. Use the

following sections to determine loading and external compo-

nents.

LDO LOADING CAPABILITY

The LDOs in the LP5550 can regulate to a variety of output

voltages, depending on the need of the processor. These

voltages can be programmed through the PWI. Table 1

summarizes the parameters of the LP5550 LDOs.

TABLE 1. LDO Parameters

PWI Register

Output voltage

range

Recommended

Maximum Output

Current

Dropout Voltage

(typical)

Typical Load

LDO1

R8

0.6 V – 2.2 V

100 mA

200 mV

PLL

LDO2

R7

1.5 V – 3.3 V

250 mA

150 mV

I/O

LDO3

R2

V

1

0.7 V – 1.35 V

2

50 mA

200 mV

Memory/Memory

retention

1. LDO3 tracks the switching converter output voltage (V

2. LDO3 regulates at the set memory retention voltage when the LP5550 is in shutdown state.

LDO OUTPUT CAPACITOR

The output capacitor sets a low frequency pole and a high

frequency zero in the control loop of an LDO. The capaci-

tance and the equivalent series resistance (ESR) of the

capacitor must be within a specified range to meet stability

requirements. The LDOs in the LP5550 are designed to be

used with ceramic output capacitors. The dielectric should

be X5R, X7R, or comparable material to maintain proper

tolerances. Use the following table to choose a suitable

output capacitor:

TABLE 2. Output Capacitor Selection Guide

Output Capacitance Range

(Recommended Typical Value)

ESR range

LDO1

1 µF – 20 µF (2.2 µF)

5 mohm – 500 mohm

LDO2

2 µF – 20 µF (4.7 µF)

5 mohm – 500 mohm

LDO3

0.7 µF – 2.2 µF (1.0 µF)

5 mohm– 500 mohm

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