
LM3661
450mA Subminiature, Micropower Step-Down DC-DC
Converter for Ultra Low-Voltage Circuits
General Description
The LM3661 step-down DC-DC converter is optimized for
powering ultra-low voltage circuits from a single Lithium-Ion
cell. The device provides two pin-selectable output voltages.
See ordering information for a list of voltage options avail-
able . This allows adjustment for DSP or CPU voltage op-
tions, as well as dynamic output voltage switching for re-
duced
power
consumption.
Internal
synchronous
rectification provides high efficiency (92% typ. at 1.35V
OUT).
The LM3661 offers superior features and performance for
mobile phones and similar portable applications. Pin-
selectable PWM and Linear modes provide improved system
control for maximizing battery life. During full-load, fixed
frequency PWM operation reduces interference in RF and
data acquisition applications by minimizing noise harmonics
at sensitive IF and sampling frequencies. The SYNC/MODE
input allows synchronization of the switching frequency in a
range of below 500 kHz to 750 kHz to prevent noise from
intermodulation with system frequencies. Linear operation
reduces quiescent current to 29 A (typ.) during system
standby for extended battery life, while supplying up to
15 mA. Shutdown turns the device off and reduces battery
consumption to 0.5 A (typ.). This device offers a selectable
over Current Limit to protect a variety of inductors.
The LM3661 is available in a 10 pin micro SMD package.
This packaging uses National’s chip-scale micro SMD tech-
nology and offers the smallest possible size. A high (600
kHz) switching frequency allows use of tiny surface-mount
components; only three are required — an inductor and two
ceramic capacitors.
Features
n
Operates from a single Li-ION cell
n
Pin selectable output voltages
n
Pin selectable Inductor Current Limit
n ±
3% output voltage precision in PWM mode
n
Miniature 10-pin micro SMD package
n
Only three tiny surface-mount external components
required
n
Uses small ceramic capacitors
n
8 mV typ. PWM output voltage ripple
n
Internal synchronous rectification for high efficiency
(92% at 2.7 V
IN, 1.35 VOUT)
n
29 A typ. quiescent current (Linear mode)
n
0.5 A typ. shutdown current
n
SYNC/MODE input for frequency synchronization from
500 kHz to 750 kHz
n
Current and Thermal overload protection
n
High gain control loop with internal compensation
n
Up to 450mA I
OUT capability for LM3661-1.35/1.4
n
Up to TBDmA I
OUT capability for LM3661-1.25
Applications
n
Mobile phones
n
Hand-Held radios
n
Personal Digital Assistants
n
Palm-top PC’s and Pocket PC’s
n
Portable Instruments
n
Battery Powered Device
Typical Application Circuit
20098802
May 2005
LM3661
450mA
Subminiature,
Micropower
Step-Down
DC-DC
Converter
for
Ultra
Low-V
oltage
Circuits
2005 National Semiconductor Corporation
DS200988
www.national.com