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PDF LM26001 Data sheet ( Hoja de datos )
| Número de pieza | LM26001 | |
| Descripción | Switching Regulator | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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PRELIMINARY
May 2006
LM26001
1.5A Switching Regulator with High Efficiency Sleep
Mode
General Description
The LM26001 is a switching regulator designed for the high
efficiency requirements of applications with stand-by modes.
The device features a low-current sleep mode to maintain
efficiency under light-load conditions and current-mode con-
trol for accurate regulation over a wide input voltage range.
Quiescent current is reduced to 10 µA typically in shutdown
mode and less than 40 µA in sleep mode. Forced PWM
mode is also available to disable sleep mode.
The LM26001 can deliver up to 1.5A of continuous load
current with a fixed current limit, through the internal
N-channel switch. The part has a wide input voltage range of
4.0V to 38V and can operate with input voltages as low as
3V during line transients.
Operating frequency is adjustable from 150 kHz to 500 kHz
with a single resistor and can be synchronized to an external
clock.
Other features include Power good, adjustable soft-start,
enable pin, input under-voltage protection, and an internal
bootstrap diode for reduced component count.
Features
n High efficiency sleep mode
n 40 µA typical Iq in sleep mode
n 10 µA typical Iq in shutdown mode
n 3.0V minimum input voltage
n 4.0V to 38V continuous input range
n 1.5% reference accuracy
n Cycle-by-cycle current limit
n Adjustable Frequency (150 kHz to 500 kHz)
n Synchronizable to an external clock
n Power Good Flag
n Forced PWM function
n Adjustable Soft-start
n TSSOP-16 exposed pad package
n Thermal Shut Down
Applications
n Automotive Telematics
n Navigation systems
n In-Dash Instrumentation
n Battery Powered Applications
n Stand-by power for home gateways/set-top boxes
Typical Application Circuit
© 2006 National Semiconductor Corporation DS201794
20179401
www.national.com
1 page  
Typical Performance Characteristics Unless otherwise specified the following conditions apply: Vin
= 12V, TJ = 25˚C.
VFB vs Temperature
VFB vs Vin
(IDC = 300 mA)
20179403
IQ and IVBIAS vs Temperature (Sleep Mode)
20179405
IQ and IVBIAS vs Temperature (PWM Mode)
20179404
Normalized Switching Frequency vs Temperature
(300kHz)
20179406
UVLO Threshold vs Temperature (VDD= VIN)
20179416
5
20179417
www.national.com
5 Page 
Design Information (Continued)
FIGURE 7. Example Circuit
1.5A Max, 305 kHz
20179430
SETTING OUTPUT VOLTAGE
The output voltage is set by the ratio of a voltage divider at
the FB pin as shown in the typical application. The resistor
values can be determined by the following equation:
Remember that inductor value also affects the sleep mode
threshold as shown in Figure 3.
When choosing the inductor, the saturation current rating
must be higher than the maximum peak inductor current and
the RMS current rating should be higher than the maximum
load current. Peak inductor current, Ipeak, is calculated as:
Where Vfb = 1.234V typically.
A maximum value of 150kΩ is recommended for the sum of
R1 and R2.
As input voltage decreases towards the nominal output volt-
age, the LM26001 can skip up to seven off-pulses as de-
scribed in the Low Vin Operation section. In low output
voltage applications, if the on-time reaches TonMIN, the de-
vice will skip on-pulses to maintain regulation. There is no
limit to the number of pulses that are skipped. In this mode of
operation, however, output ripple voltage may increase
slightly.
INDUCTOR
The output inductor should be selected based on inductor
ripple current. The amount of inductor ripple current com-
pared to load current, or ripple content, is defined as Iripple/
Iload. Ripple content should be less than 40%. Inductor
ripple current, Iripple, can be calculated as shown below:
Larger ripple content increases losses in the inductor and
reduces the effective current limit.
Larger inductance values result in lower output ripple voltage
and higher efficiency, but a slightly degraded transient re-
sponse. Lower inductance values allow for smaller case
size, but the increased ripple lowers the effective current
limit threshold.
For example, at a maximum load of 1.5A and a ripple content
of 40%, peak inductor current is equal to 1.8A which is safely
below the minimum current limit of 1.85A. By increasing the
inductor size, ripple content and peak inductor current are
lowered, which increases the current limit margin.
The size of the output inductor can also be determined using
the desired output ripple voltage, Vrip. The equation to de-
termine the minimum inductance value based on Vrip is as
follows:
Where Re is the ESR of the output capacitors, and Vrip is a
peak-to-peak value. This equation assumes that the output
capacitors have some amount of ESR. It does not apply to
ceramic output capacitors.
If this method is used, ripple content should still be verified to
be less than 40%.
OUTPUT CAPACITOR
The primary criterion for selecting an output capacitor is
equivalent series resistance, or ESR.
ESR (Re) can be selected based on the requirements for
output ripple voltage and transient response. Once an induc-
tor value has been selected, ripple voltage can be calculated
for a given Re using the equation above for Lmin. Lower
ESR values result in lower output ripple.
11 www.national.com
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LM26001.PDF ] | |
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