EN1

Buck1

LDO1 & 2

Buck2

85%

Timings shown are typical.

150 s

EN2

110 s

60 s

LP3905

SNVS374D – JUNE 2006 – REVISED MAY 2013

www.ti.com

FUNCTIONAL DESCRIPTION

POWER UP/DOWN PROCEDURE

The LP3905 Bucks and LDOs are powered UP/DOWN with 2 control pins, EN1 and EN2. In order for the enable

pins to operate, VIN1 and VIN2 should be set to a voltage level higher than VUVLO_R (specified in electrical

characteristic). Once enabled, EN1 will turn on Buck1, LDO1 and LDO2. EN2 can independently be used to

enable Buck2. Figure 5 illustrates the power UP/DOWN timing sequence of the LP3905 blocks for VEN≥ VIH (min)

(enable) and VEN≤VIL (max) (disable).

Both linear regulators have active pulldowns when the outputs are disabled.

Figure 5. LP3905 Power Up and Power Down Timing Sequence

EN1 and EN2 can be controlled fully independently.

LDOs will be turned on only after Buck1 is powered up. LDOs are powered on simultaneously.

In case EN1 and EN2 are enabled at the same time, power up of Buck2 is delayed by 50µs in order to minimize

the inrush current from the battery.

When EN1 and EN2 are disabled, the relevant output voltages are turned off.

DC/DC BUCK REGULATORS

The LP3905 Buck regulators are high efficiency step down DC-DC switching converters used for delivering a

constant voltage from either a single Li-Ion or three cell NiMH/NiCd battery to portable devices such as cell

phones and PDAs. Using a voltage mode architecture with synchronous rectification, the Buck Regulators have

the ability to deliver up to 600 mA depending on the input voltage, output voltage, ambient temperature and the

inductor chosen.

There are three modes of operation depending on the current required - PWM, PFM, and shutdown. The

standard device operates in PWM mode at load currents of approximately 80 mA or higher, having voltage

tolerance of ±4% with 90% efficiency or better. Lighter load currents cause the device to automatically switch into

PFM for reduced current consumption and a longer battery life. Shutdown mode turns off the device, offering the

lowest current consumption . A fixed mode device is also available which is fixed in PWM mode for both low and

high load currents.

An adjustable voltage version is also avalable for which the output voltage can be selected by using two external

resistors at each of the two buck outputs.

8

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