LM2771 datasheet(7/10 Pages) NSC

7 / 10 page

Operation Description

OVERVIEW

The LM2771 is a switched capacitor converter that produces

a regulated, low voltage output. The core of the part is a

highly efficient charge pump that utilizes fixed frequency

pre-regulation and Pulse Frequency Modulation to minimize

ripple and power losses over wide input voltage and output

current ranges. A description of the principal operational

characteristics of the LM2771 is detailed in the Circuit De-

scription, and Efficiency Performance sections. These

sections refer to details in the Block Diagram.

CIRCUIT DESCRIPTION

The core of the LM2771 is a two-phase charge pump con-

trolled by an internally generated non-overlapping clock. The

charge pump operates by using an external flying capacitor,

C1, to transfer charge from the input to the output. At input

voltages below 3.5V (typ.) the LM2771 operates in a "pass

mode", with the input current being equal to the load current.

At input voltages above 3.5V (typ.) the part utilizes a gain of

⁄2, resulting in an input current equal to half the load current.

The two phases of the switched capacitor switching cycle will

be referred to as the "charge phase" and the "discharge

phase". During the charge phase, the flying capacitor is

charged by the input supply. After half of the switching cycle

[ t = 1/(2xF

SW) ], the LM2771 switches to the discharge

phase. In this configuration, the charge that was stored on

the flying capacitor in the charge phase is transferred to the

output.

The LM2771 uses fixed frequency pre-regulation to regulate

the output voltage to 1.5V during moderate to high load

currents. The input and output connections of the flying

capacitor is made with internal MOS switches. Pre-

regulation limits the gate drive of the MOS switch connected

between the voltage input and the flying capacitor. Control-

ling the on resistance of this switch limits the amount of

charge transferred into and out of the flying capacitor during

the charge and discharge phases, and in turn helps to keep

the output ripple very low.

When output currents are low (<30mA typ.), the LM2771

automatically switches to a low-ripple Pulse Frequency

Modulation (PFM) form of regulation. In PFM mode, the

flying capacitor stays in the discharge phase until the output

voltage drops below a predetermined trip point. When this

occurs, the flying capacitor switches back to the charge

phase. After being charged, the flying capacitor repeats the

process of staying in the discharge phase and switching to

the charge phase when necessary.

EFFICIENCY PERFORMANCE

Charge-pump efficiency is derived in the following two ideal

equations (supply current and other losses are neglected for

simplicity):

IN =GxIOUT

E=(V

OUT xIOUT)÷(VIN xIIN)=VOUT ÷(GxVIN)

In the equations, G represents the charge pump gain. Effi-

ciency is at its highest as GxV

IN approaches VOUT. Refer to

the efficiency graph in the Typical Performance Character-

istics section for detailed efficiency data. The transition

between Pass mode and the gain of 1⁄2 is clearly distin-

guished by the sharp discontinuity in the efficiency curve.

SHUTDOWN

The LM2771 is in shutdown mode when the voltage on the

enable pin (EN) is logic-low. In shutdown, the LM2771 draws

virtually no supply current. When in shutdown, the output of

the LM2771 is completely disconnected from the input. The

internal feedback resistors will pull the output voltage down

to 0V.

SOFT START

The LM2771 employs soft start circuitry to prevent excessive

input inrush currents during startup. At startup, the output

voltage gradually rises from 0V to the nominal output volt-

age. This occurs in 150µs (typ.). Soft-start is engaged when

the part is enabled, including situations where voltage is

established simultaneously on the V

IN and EN pins.

THERMAL SHUTDOWN

Protection from damage related to overheating is achieved

with a thermal shutdown feature. When the junction tem-

perature rises to 150oC (typ.), the part switches into shut-

down mode. The LM2771 disengages thermal shutdown

when the junction temperature of the part is reduced to

140oC (typ.). Due to the high efficiency of the LM2771,

thermal shutdown and/or thermal cycling should not be en-

countered when the part is operated within specified input

voltage, output current, and ambient temperature operating

ratings. If thermal cycling is seen under these conditions, the

most likely cause is an inadequate PCB layout that does not

allow heat to be sufficiently dissipated out of the LLP pack-

age.

CURRENT LIMIT PROTECTION

The LM2771 charge pump contains current limit protection

circuitry that protects the device during V

OUT fault conditions

where excessive current is drawn. Output current is limited

to 500mA (typ).

Application Information

RECOMMENDED CAPACITOR TYPES

The LM2771 requires 3 external capacitors for proper opera-

tion. Surface-mount multi-layer ceramic capacitors are rec-

ommended. These capacitors are small, inexpensive and

have very low equivalent series resistance (ESR,

≤ 15mΩ

typ.). Tantalum capacitors, OS-CON capacitors, and alumi-

num electrolytic capacitors generally are not recommended

for use with the LM2771 due to their high ESR, as compared

to ceramic capacitors.

For most applications, ceramic capacitors with an X7R or

X5R temperature characteristic are preferred for use with the

LM2771. These capacitors have tight capacitance tolerance

(as good as ±10%) and hold their value over temperature

(X7R: ±15% over -55oC to 125oC; X5R: ±15% over -55oCto

85oC).

Capacitors with a Y5V or Z5U temperature characteristic are

generally not recommended for use with the LM2771. These

types of capacitors typically have wide capacitance toler-

ance (+80%, -20%) and vary significantly over temperature

(Y5V: +22%, -82% over -30oCto+85oC range; Z5U: +22%,

-56% over +10oCto+85oC range). Under some conditions, a

1µF-rated Y5V or Z5U capacitor could have a capacitance

as low as 0.1µF. Such detrimental deviation is likely to cause

Y5V and Z5U capacitors to fail to meet the minimum capaci-

tance requirements of the LM2771.

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