LM3686

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SNVS520E – AUGUST 2008 – REVISED MAY 2013

Operation Description

DEVICE INFORMATION

The LM3686 incorporates a high efficiency synchronous switching step-down DC-DC converter, a very low

dropout linear regulator (LILO), and ultra low noise linear regulator.

The DC-DC converter delivers a constant voltage from a single Li- Ion battery and input voltage rails from 2.7V to

5.5V to portable devices such as cell phones and PDAs. Using a voltage mode architecture with synchronous

rectification, it has the ability to deliver up to 600 mA load current (when not powering the LILO) depending on

the input voltage, output voltage, ambient temperature and the inductor chosen.

the DC-DC converter is used (post regulation) with a maximum load current of 350 mA.

current of 300 mA.

Three enable pins allow the independent control of the three outputs. Shutdown mode turns off the device,

Besides the shutdown feature, for the DC-DC converter there are two more modes of operation depending on the

current required:

- PWM (Pulse Width Modulation), and

- PFM (Pulse Frequency Modulation).

The device operates in PWM mode at load current of approximately 80 mA or higher. Lighter load current cause

battery life.

Additional features include soft-start, startup mode of the linear regulator, under-voltage protection, current

overload protection, and over-temperature protection.

An internal reference generates a 1.8V biasing an internal resistive divider to create a reference voltage range

from 0.7V to 1.8V (in 50 mV steps) for the LILO and the 0.5V reference used for the DC-DC converter. The ultra

low noise linear regulator also has internal reference that generates a 1.8V biasing for a internal resistor divider.

Thus, creating a reference voltage ranging from 1.5V to 3.3V

Post Regulation Note:

In the case that the DC-DC converter is switched off while the Linear Regulator is still enabled, the LILO can still

maximum current is 50 mA when this small NMOS is ON. If higher current > 50 mA is desired the following

condition must be done:

1) EN_DC = HIGH

When the condition is met, the LILO transitions to the large NMOS and can support up to 350 mA.

DC-DC CONVERTER OPERATION

During the first part of each switching cycle, the control block in the LM3686 turns on the internal PFET switch.

energy in the magnetic field.

During the second part of each cycle, the controller turns the PFET switch off, blocking current flow from the

input, and then turns the NFET synchronous rectifier on. The inductor draws current from ground through the

L).

The output filter stores charge when the inductor current is high, and releases it when low, smoothing the voltage

across the load.

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