BYPASS

LP3985

IN

OUT

EN

3(A1)

5(C1)

4(A3)

2(B2)

1µF

1(C3)

*

1µF

Pin Numbers in parenthesis indicate DSBGA package.

* Optional Noise Reduction Capacitor.

LP3985

www.ti.com

SNVS087AE – OCTOBER 2000 – REVISED MAY 2015

8 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

8.1 Application Information

The LP3985 can provide 150-mA output current with 2.5-V to 6-V input. It is stable with a small 1-µF ±30%

ceramic or high-quality tantalum output capacitor. The DSBGA requires the smallest possible PC board

area – the total application circuit area can be less than 2 mm x 2.5 mm, a fraction of a 1206 case size. An

optional external bypass capacitor reduces the output noise without slowing down the load transient

response. Fast startup time is achieved by utilizing an internal power-on circuit that actively pre-charges the

bypass capacitor. Typical output noise is 30 µVRMS at frequencies from 10 Hz to 100 kHz. Typical power

supply rejection is 50 dB at 1 kHz.

8.2 Typical Application

Figure 37. LP3985 Typical Application

8.2.1 Design Requirements

DESIGN PARAMETERS

VALUE

Input voltage

4.2 V, ±10% provided by the DC-DC converter switching at 1 MHz

Output voltage

3 V, ±5%

Output current

150 mA (maximum)

RMS noise, 10 Hz to100 kHz

30

μVRMS

PSRR at 1 kHz

50 dB

8.2.2 Detailed Design Procedure

8.2.2.1 External Capacitors

Like any low-dropout regulator, the LP3985 requires external capacitors for regulator stability. The LP3985 is

specifically designed for portable applications requiring minimum board space and smallest components. These

capacitors must be correctly selected for good performance.

8.2.2.2 Input Capacitor

An input capacitance of approximately 1 µF is required between the LP3985 input pin and ground (the amount of

the capacitance may be increased without limit).

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