LM3880 datasheet(13/20 Pages) NSC

13 / 20 page

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 sin-

gle Li- Ion battery and input voltage rails from 2.7V to 5.5V to

portable devices such as cell phones and PDAs. Using a volt-

age 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 linear regulator delivers a constant voltage biased from

IN_LILO power input typically the output voltage of the DC-DC

converter is used (post regulation) with a maximum load cur-

rent of 350 mA.

The other linear regulator delivers a constant voltage biased

from V

IN_LDO power input - with a maximum load current of

300 mA.

Three enable pins allow the independent control of the three

outputs. Shutdown mode turns off the device, offering the

lowest current consumption (I

SHUTDOWN = 2.5 µA typ).

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 approx-

imately 80 mA or higher. Lighter load current cause the device

to automatically switch into PFM for reduced current con-

sumption (I

Q_VBATT = 28 µA typ) and a longer battery life.

Additional features include soft-start, startup mode of the lin-

ear regulator, under-voltage protection, current overload pro-

tection, and over-temperature protection.

An internal reference generates a 1.8V biasing an internal re-

sistive 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 reg-

ulator also has internal reference that generates a 1.8V bias-

ing for a internal resistor divider. Thus, creating a reference

voltage ranging from 1.5V to 3.3V

The Under-voltage lockout feature enables the device to start-

up once V

BATT has reached 2.65V typically and turns the

device off if V

BATT drops below 2.41V typically.

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 support up

to 50 mA. The linear regulator LILO is turned on via a small

NMOS device supplied by V

IN_LDO . The 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. This allows

current to flow from the input V

BATT through the switch pin SW

and the inductor to the output filter capacitor and load. The

inductor limits the current to a ramp with a slope of (V

BATT -

OUT_DCDC) / L, by storing 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 NFET to the output filter ca-

pacitor and load, which ramps the inductor current down with

a slope of (- V

OUT_DCDC / L).

The output filter stores charge when the inductor current is

high, and releases it when low, smoothing the voltage across

the load.

The output voltage is regulated by modulating the PFET

switch on time to control the average current sent to the load.

The effect is identical to sending a duty-cycle modulated rect-

angular wave formed by the switch and synchronous rectifier

at the SW pin to a low-pass filter formed by the inductor and

output filter capacitor. The output voltage is equal to the av-

erage voltage at the SW pin.

PWM Operation

During PWM (Pulse Width Modulation) operation the con-

verter operates as a voltage-mode controller with input volt-

age feed forward. This allows the converter to achieve good

load and line regulation. The DC gain of the power stage is

proportional to the input voltage. To eliminate this dependen-

cy, feed forward inversely proportional to the input voltage is

introduced.

While in PWM mode, the output voltage is regulated by

switching at a constant frequency and then modulating the

energy per cycle to control power to the load. At the beginning

of each clock cycle the PFET switch is turned on and the in-

ductor current ramps up until the duty-cycle-comparator trips

and the control logic turns off the switch. The current limit

comparator can also turn off the switch in case the current

limit of the PFET is exceeded. Then the NFET switch is turned

on and the inductor current ramps down. The next cycle is

initiated by the clock turning off the NFET and turning on the

PFET.

30025509

FIGURE 4. Typical PWM Operation

Internal Synchronous Rectification

While in PWM mode, the DC-DC converter uses an internal

NFET as a synchronous rectifier to reduce rectifier forward

voltage drop and associated power loss. Synchronous recti-

fication provides a significant improvement in efficiency

whenever the output voltage is relatively low compared to the

voltage drop across an ordinary rectifier diode.

LM3880 Datasheet(PDF) 13 Page - National Semiconductor (TI)

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