lm2618 National Semiconductor Corporation, lm2618 Datasheet - Page 9

• Manufacturer Part Number: lm2618
• Description: 400ma Sub-miniature, High Efficiency, Synchronous Pwm & Pfm Programmable Dc-dc Converter
• Manufacturer: National Semiconductor Corporation

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
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
-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
OUT
)/L, by storing energy in a magnetic field. During the
FIGURE 1. Typical Operating Circuit
IN
9
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.
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
OUT
/L. If the inductor
www.national.com