LM2842

Manufacturer Part NumberLM2842
ManufacturerTexas Instruments
LM2842 datasheet
 

Specifications of LM2842

Iout(max)(a)0.6Vin(min)(v)4.5
Vin(max)(v)42Vout(min)(v)0.765
Vout(max)(v)34TopologyBuck
Pin/package6SOTRegulated Outputs(#)1
Frequency(min)(khz)550Frequency(max)(khz)1250
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output capacitor for the same output voltage ripple require-
ment. A reasonable value is setting the ripple current to be
30% of the DC output current. Since the ripple current in-
creases with the input voltage, the maximum input voltage is
always used to determine the inductance. The DC resistance
of the inductor is a key parameter for the efficiency. Lower DC
resistance is available with a bigger winding area. A good
tradeoff between the efficiency and the core size is letting the
inductor copper loss equal 2% of the output power. See
AN-1197 for more information on selecting inductors. A good
starting point for most applications is a 10 µH to 22 µH with
1.1A or greater current rating for the LM2842 or a 0.7A or
greater current rating for the LM2840/41. Using such a rating
will enable the LM2840/1/2 to current limit without saturating
the inductor. This is preferable to the LM2840/1/2 going into
thermal shutdown mode and the possibility of damaging the
inductor if the output is shorted to ground or other longterm
overload.
OUTPUT CAPACITOR
The selection of C
is driven by the maximum allowable
OUT
output voltage ripple. The output ripple in the constant fre-
quency, PWM mode is approximated by: V
(ESR+(1/(8f
C
))) The ESR term usually plays the dom-
SW
OUT
inant role in determining the voltage ripple. Low ESR ceramic
capacitors are recommended. Capacitors in the range of 22
µF-100 µF are a good starting point with an ESR of 0.1Ω or
less.
BOOTSTRAP CAPACITOR
A 0.15 µF ceramic capacitor or larger is recommended for the
bootstrap capacitor (C
). For applications where the input
BOOT
voltage is less than twice the output voltage a larger capacitor
is recommended, generally 0.15 µF to 1 µF to ensure plenty
of gate drive for the internal switches and a consistently low
R
.
DSON
SOFT-START COMPONENTS
The LM2840/1/2 has circuitry that is used in conjunction with
the SHDN pin to limit the inrush current on start-up of the DC/
DC switching regulator. The SHDN pin in conjunction with a
RC filter is used to tailor the soft-start for a specific application.
When a voltage applied to the SHDN pin is between 0V and
up to 2.3V it will cause the cycle by cycle current limit in the
power stage to be modulated for minimum current limit at 0V
up to the rated current limit at 2.3V. Thus controlling the output
rise time and inrush current at startup. The resistor value
should be selected so the current sourced into the SHDN pin
will be greater then the leakage current of the SHDN pin (1.5
µA ) when the voltage at SHDN is equal or greater then 2.3V.
SHUTDOWN OPERATION
The SHDN pin of the LM2840/1/2 is designed so that it may
be controlled using 2.3V or higher logic signals. If the shut-
down function is not to be used the SHDN pin may be tied to
V
. The maximum voltage to the SHDN pin should not ex-
IN
ceed 42V. If the use of a higher voltage is desired due to
system or other constraints it may be used, however a 100
kΩ or larger resistor is recommended between the applied
voltage and the SHDN pin to protect the device.
SCHOTTKY DIODE
The breakdown voltage rating of the diode (D1) is preferred
to be 25% higher than the maximum input voltage. The cur-
rent rating for the diode should be equal to the maximum
output current for best reliability in most applications. In cases
where the input voltage is much greater than the output volt-
age the average diode current is lower. In this case it is
possible to use a diode with a lower average current rating,
approximately (1-D)I
= I
RIPPLE
RIPPLE
should be higher than the maximum load current. A 0.5A to
1A rated diode is a good starting point.
LAYOUT CONSIDERATIONS
To reduce problems with conducted noise pick up, the ground
side of the feedback network should be connected directly to
the GND pin with its own connection. The feedback network,
resistors R1 and R2, should be kept close to the FB pin, and
away from the inductor to minimize coupling noise into the
feedback pin. The input bypass capacitor C
close to the V
IN
which effects input voltage ripple of the IC. The inductor L1
should be placed close to the SW pin to reduce magnetic and
electrostatic noise. The output capacitor, C
placed close to the junction of L1 and the diode D1. The L1,
D1, and C
trace should be as short as possible to reduce
OUT
conducted and radiated noise and increase overall efficiency.
The ground connection for the diode, C
be as small as possible and tied to the system ground plane
in only one spot (preferably at the C
imize conducted noise in the system ground plane. For more
detail on switching power supply layout considerations see
Application Note AN-1149: Layout Guidelines for Switching
Power Supplies.
9
, however the peak current rating
OUT
must be placed
IN
pin. This will reduce copper trace resistance
should be
OUT
, and C
should
IN
OUT
ground point) to min-
OUT
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