lm2798mm-2.0 National Semiconductor Corporation, lm2798mm-2.0 Datasheet - Page 12

• Manufacturer Part Number: lm2798mm-2.0
• Description: 120ma High Efficiency Step-down Switched Capacitor Voltage Converter With Voltage Monitoring
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
Capacitors with a Y5V or Z5U temperature characteristic are
generally not recommended for use with the LM2797/98.
These types of capacitors typically have wide capacitance
tolerance (+80%, -20%) and vary significantly over tempera-
ture (Y5V: +22%, -82% over -30
+22%, -56% over +10
ditions, a 1µF-rated Y5V or Z5U capacitor could have a
capacitance as low as 0.1µF. Such detrimental deviation is
likely to cause these Y5V and Z5U capacitors to fail to meet
the minimum capacitance requirements of the LM2797/98.
The table below lists some leading ceramic capacitor manu-
facturers.
OUTPUT CAPACITOR
The output capacitor of the LM2797/98 greatly affect perfor-
mance of the circuit. In typical high-current applications, a
10µF low-ESR (ESR = equivalent series resistance) ceramic
capacitor is recommended. For lighter loads, the output
capacitance may be reduced (as low as 1µF for output
currents ≤ 60mA is usually acceptable). The performance of
the part should be evaluated with special attention paid to
efficiency and output ripple to ensure the capacitance cho-
sen on the output yields performance suitable for the appli-
cation. In extreme cases, excessive ripple could cause con-
trol loop instability, severely affecting the performance of the
part. If excessive ripple is present, the output capacitance
should be increased.
The ESR of the output capacitor affects charge pump output
resistance, which plays a role in determining output current
capability. Both output capacitance and ESR affect output
voltage ripple (See Output Voltage Ripple section, above).
For these reasons, a low-ESR X7R/X5R ceramic capacitor is
the capacitor of choice for the LM2797/98 output.
FLYING CAPACITORS
The flying capacitors (C
input to the output, and determine the strength of the charge
pump: the larger the capacitance, the greater the output
current capability. If capacitors are too small, the LM2797/98
could spend excessive amount of time gain hopping: de-
creasing efficiency, increasing output voltage ripple, and
possibly impeding the ability of the part to regulate. On the
other hand, if the flying capacitors are too large they could
potentially overwhelm the output capacitor, resulting in in-
creased output voltage ripple.
Vishay-Vitramon
Manufacturer
Taiyo-Yuden
Murata
TDK
AVX
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C to +85
1
and C
www.component.tdk.com
Contact Information
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C range). Under some con-
2
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www.murata.com
www.vishay.com
) transfer charge from the
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C to +85
(Continued)
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C range; Z5U:
12
Low-ESR ceramic capacitors with X7R or X5R temperature
characteristic are strongly recommended for use here. The
flying capacitors C1 and C2 should be identical. As a general
rule, the capacitance value of each flying capacitor should
be 1/10th that of the output capacitor. ESR should be as low
as possible to minimize the output resistance of the charge
pump and give maximum output current capability. Polarized
capacitor (tantalum, aluminum electrolytic, etc.) must not be
used for the flying capacitors, as they could become reverse-
biased upon start-up of the LM2797/98.
INPUT CAPACITOR
The input capacitor (C
quick transfer of charge from the supply to the flying capaci-
tors during the charge phase of operation. The input capaci-
tor helps to keep the input voltage from drooping at the start
of the charge phase when the flying capacitor is connected
to the input, and helps to filter noise on the input pin that
could adversely affect sensitive internal analog circuitry bi-
ased off the input line. An X7R/X5R ceramic capacitor is
recommended for use. As a general recommendation, the
input capacitor should be chosen to match the output capaci-
tor.
POWER DISSIPATION
LM2797/98 power dissipation will, typically, not be much of a
concern in most applications. Derating to accommodate self-
heating will rarely be required due to the high efficiency of
the part. Peak power dissipation (P
tions is seen with the LM2798-1.5 operating at V
and I
ratings). Under these conditions, the power efficiency (E) of
the LM2798-1.5 is 54% (typ.). Assuming a typical junction-
to-ambient thermal resistance (θ
of 220˚C/Watt, the junction temperature (T
calculated below for a part operating at the maximum rated
ambient temperature (T
P
T
Even under these peak power dissipation and ambient tem-
perature conditions, the junction temperature of the LM2798-
1.5 is below the maximum operating rating of 125˚C.
As an additional note, the ambient temperature operating
rating range listed in the specifications is provided merely for
convenience. The LM2797/98 may be operated outside this
rating, so long as the junction temperature of the device
does not exceed the maximum operating rating of 125˚C.
J
D
= T
= P
= (P
= [(1/E) - 1] x P
= [(1/64%) - 1] x 1.5V x 120mW
= 153mW
= 85˚C + (.153W x 220˚C/W)
=119˚C
OUT
A
IN
= (P
OUT
- P
= 120mA (conditions limited to valid operating
/E) - P
OUT
D
x θ
JA
OUT
)
OUT
IN
A
) is a reservoir of charge that aids a
) of 85˚C.
JA
) for the MSOP package
D
) of all LM2797/98 op-
J
) of the part is
IN
= 5.5V