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PDF LM3670MF-1.6 Data sheet ( Hoja de datos )
| Número de pieza | LM3670MF-1.6 | |
| Descripción | Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits | |
| Fabricantes | National Semiconductor | |
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December 2004
LM3670
Miniature Step-Down DC-DC Converter for Ultra Low
Voltage Circuits
General Description
The LM3670 step-down DC-DC converter is optimized for
powering ultra-low voltage circuits from a single Li-Ion cell or
3 cell NiMH/NiCd batteries. It provides up to 350 mA load
current, over an input voltage range from 2.5V to 5.5V. There
are several different fixed voltage output options available as
well as an adjustable output voltage version.
The device offers superior features and performance for
mobile phones and similar portable applications with com-
plex power management systems. Automatic intelligent
switching between PWM low-noise and PFM low-current
mode offers improved system control. During full-power op-
eration, a fixed-frequency 1 MHz (typ). PWM mode drives
loads from ∼70 mA to 350 mA max, with up to 95% efficiency.
Hysteretic PFM mode extends the battery life through reduc-
tion of the quiescent current to 15 µA (typ) during light
current loads and system standby. Internal synchronous rec-
tification provides high efficiency (90 to 95% typ. at loads
between 1 mA and 100 mA). In shutdown mode (Enable pin
pulled low) the device turns off and reduces battery con-
sumption to 0.1 µA (typ.).
The LM3670 is available in a SOT23-5 package. A high
switching frequency - 1 MHz (typ) - allows use of tiny
surface-mount components. Only three external surface-
mount components, an inductor and two ceramic capacitors,
are required.
Features
n VOUT = adj (.7V min), 1.2, 1.5, 1.6, 1.8, 1.875, 2.5, 3.3V
n 2.5V ≤ VIN ≤ 5.5V
n 15 µA typical quiescent current
n 350 mA maximum load capability
n 1 MHz PWM fixed switching frequency (typ.)
n Automatic PFM/PWM mode switching
n Available in fixed output voltages as well as an
adjustable version
n SOT23-5 package
n Low drop out operation - 100% duty cycle mode
n Internal synchronous rectification for high efficiency
n Internal soft start
n 0.1 µA typical shutdown current
n Operates from a single Li-Ion cell or 3 cell NiMH/NiCd
batteries
n Only three tiny surface-mount external components
required (one inductor, two ceramic capacitors)
n Current overload protection
Applications
n Mobile phones
n Hand-Held Radios
n Personal Digital Assistants
n Palm-top PCs
n Portable Instruments
n Battery Powered Devices
Typical Application
20075801
FIGURE 1. 1.8V - Fixed Output Voltage - Typical Application Circuit
© 2004 National Semiconductor Corporation DS200758
www.national.com
1 page  
Electrical Characteristics Limits in standard typeface are for TJ = 25˚C. Limits in boldface type apply over
the full operating junction temperature range (−40˚C ≤ TJ ≤ +125˚C). Unless otherwise noted VIN = 3.6V, VOUT = 1.8V,
IO=150mA, EN=VIN (Continued)
Symbol
VIH
VIL
IEN
FOSC
Parameter
Logic High Input
Logic Low Input
Enable (EN) Input Current
Internal Oscillator Frequency
Condition
PWM Mode
Min Typ
1.3
0.01
550 1000
Max
0.4
1
1300
Units
V
V
µA
kHz
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation of
the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the
Electrical Characteristics tables.
Note 2: All voltages are with respect to the potential at the GND pin.
Note 3: The Human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. The machine model is a 200 pF capacitor discharged
directly into each pin. MIL-STD-883 3015.7
5 www.national.com
5 Page 
Operation Description (Continued)
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 capacitor
and load, which ramps the inductor current down with a
slope of
The output filter stores charge when the inductor current is
high, and releases it when low, smoothing the voltage across
the load.
PWM OPERATION
During PWM operation the converter operates as a voltage-
mode controller with input voltage feed forward. This allows
the converter to achieve excellent load and line regulation.
The DC gain of the power stage is proportional to the input
voltage. To eliminate this dependence, feed forward in-
versely proportional to the input voltage is introduced.
Internal Synchronous Rectification
While in PWM mode, the LM3670 uses an internal NFET as
a synchronous rectifier to reduce rectifier forward voltage
drop and associated power loss. Synchronous rectification
provides a significant improvement in efficiency whenever
the output voltage is relatively low compared to the voltage
drop across an ordinary rectifier diode.
Current Limiting
A current limit feature allows the LM3670 to protect itself and
external components during overload conditions PWM mode
implements cycle-by-cycle current limiting using an internal
comparator that trips at 620 mA (typ).
PFM OPERATION
At very light loads, the converter enters PFM mode and
operates with reduced switching frequency and supply cur-
rent to maintain high efficiency.
The party will automatically transition into PFM mode when
either of two conditions occurs for a duration of 32 or more
clock cycles:
A. The inductor current becomes discontinuous
B. The peak PMOS switch current drops below the IMODE
level:
During PFM operation, the converter positions the output
voltage slightly higher than the nominal output voltage during
PWM operation, allowing additional headroom for voltage
drop during a load transient from light to heavy load. The
PFM comparators sense the output voltage via the feedback
pin and control the switching of the output FETs such that the
output voltage ramps between 0.8% and 1.6% (typ) above
the nominal PWM output voltage. If the output voltage is
below the ‘high’ PFM comparator threshold, the PMOS
power switch is turned on. It remains on until the output
voltage exceeds the ‘high’ PFM threshold or the peak current
exceeds the IPFM level set for PFM mode. The peak current
in PFM mode is:
Once the PMOS power switch is turned off, the NMOS
power switch is turned on until the inductor current ramps to
zero. When the NMOS zero-current condition is detected,
the NMOS power switch is turned off. If the output voltage is
below the ‘high’ PFM comparator threshold (see Figure 5),
the PMOS switch is again turned on and the cycle is re-
peated until the output reaches the desired level. Once the
output reaches the ‘high’ PFM threshold, the NMOS switch is
turned on briefly to ramp the inductor current to zero and
then both output switches are turned off and the part enters
an extremely low power mode. Quiescent supply current
during this ‘sleep’ mode is less than 30 µA, which allows the
part to achieve high efficiencies under extremely light load
conditions. When the output drops below the ‘low’ PFM
threshold, the cycle repeats to restore the output voltage to
∼1.6% above the nominal PWM output voltage.
If the load current should increase during PFM mode (see
Figure 5) causing the output voltage to fall below the ‘low2’
PFM threshold, the part will automatically transition into
fixed-frequency PWM mode.
11 www.national.com
11 Page | ||
| Páginas | Total 17 Páginas | |
| PDF Descargar | [ Datasheet LM3670MF-1.6.PDF ] | |
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