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LM2612 Datasheet(Fiches technique) 12 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Numéro de pièce LM2612
Description  400mA Sub-miniature, Programmable, Step-Down DC-DC Converter for Ultra Low-Voltage Circuits
Télécharger  19 Pages
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Fabricant  NSC [National Semiconductor (TI)]
Site Internet  http://www.national.com
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 12 page
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Device Information
The LM2612 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 1.05V, 1.3V, 1.5V or
1.8V 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).
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. PWM mode uses syn-
chronous rectification for high efficiency: typically 91% for a
100mA load with 1.8V output, 2.8V input. 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 150µA typ to maximize battery life. Shutdown
mode turns the device off and reduces battery consumption
to 0.1µA (typ.).
The LM2612 offers good performance and a full set of fea-
tures. It is based on a current-mode switching buck
architecture for cycle-by-cycle current limiting. DC PWM
mode output voltage precision is ±2% for most output volt-
ages and ±3% for 1.05V. The SYNC/MODE input accepts an
external clock between 500kHz and 1MHz. The output volt-
age selection pins eliminate external feedback resistors.
Additional features include soft-start, current overload pro-
tection, over-voltage protection and thermal overload protec-
tion.
The LM2612 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
10uH inductor, and tiny 10uF and 22uF ceramic capacitors
for reduced board area.
Circuit Operation
Referring to
Figure 1, Figure 2, and Figure 3 the LM2612
operates as follows: During the first part of each switching
cycle, the control block in the LM2612 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
OUT)/L, by storing energy in a magnetic field. During the
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
OUT/L. If the inductor
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.
20007103
FIGURE 1. Typical Operating Circuit
www.national.com
12




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