Device Information

The LM2618 is a simple, step-down DC-DC converter opti-

mized for powering low-voltage CPUs or DSPs in cell

phones and other miniature battery powered devices. It pro-

vides pin-selectable output voltages of 1.80V, 1.83V, 1.87V

or 1.92V from a single 2.8V to 5.5V LiION battery cell. It is

designed for a maximum load capability of 400mA (300mA

for B grade). It uses synchronous rectification in both PWM

and PFM modes for high efficiency: typically 91% for a

100mA load with 1.92V output, 3.0V input, while in PWM

mode.

The device has all three of the pin-selectable operating

modes required for cell phones and other complex portable

devices. Such applications typically spend a small portion of

their time operating at full power. During full power operation,

synchronized or fixed-frequency PWM mode offers full out-

put current capability while minimizing interference to sensi-

tive IF and data acquisition circuits. These applications

spend the remainder of their time in low-current standby

operation or shutdown to conserve battery power. During

standby operation, hysteretic PFM mode reduces quiescent

current to 180µA typ to maximize battery life. Shutdown

mode turns the device off and reduces battery consumption

to 0.02µA (typ.).

The LM2618 offers good performance and a full set of fea-

tures. It is based on a current-mode switching buck

architecture. The SYNC/MODE input accepts an external

clock between 500kHz and 1MHz.

The output voltage selection pins eliminate external feed-

back resistors. Additional features include soft-start, current

overload protection, over-voltage protection and thermal

shutdown protection.

The LM2618 is constructed using a chip-scale 10-pin micro

SMD package. The micro SMD package offers the smallest

possible size for space critical applications, such as cell

phones. Required external components are only a small

10µH inductor, and tiny 10µF and 22µF ceramic capacitors

for reduced board area.

Circuit Operation

Referring to Figure 1, Figure 2, and Figure 3 the LM2618

operates as follows: During the first part of each switching

cycle, the control block in the LM2618 turns on the internal

PFET switch. This allows current to flow from the input

through the inductor to the output filter capacitor and load.

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

IN

-V

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. In response, the

inductor’s magnetic field collapses, generating a voltage that

forces current from ground through the synchronous rectifier

to the output filter capacitor and load. As the stored energy is

transferred back into the circuit and depleted, the inductor

current ramps down with a slope of V

current reaches zero before the next cycle, the synchronous

rectifier is turned off to prevent current reversal. The output

filter capacitor 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 modu-

lated rectangular wave formed by the switch and synchro-

nous rectifier to a low-pass filter created by the inductor and

output filter capacitor. The output voltage is equal to the

average voltage at the SW pin.

20036403

FIGURE 1. Typical Operating Circuit

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