lm2788mmx-1.8 National Semiconductor Corporation, lm2788mmx-1.8 Datasheet - Page 10

• Manufacturer Part Number: lm2788mmx-1.8
• Description: 120ma High Efficiency Step-down Switched Capacitor Voltage Converter
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
low-ESR ceramic capacitor is recommended on the output to
keep output voltage ripple low. Placing multiple capacitors in
parallel can reduce ripple significantly, both by increasing
capacitance and reducing ESR. When capacitors are in
parallel, ESR is in parallel as well. The effective net ESR is
determined according to the properties of parallel resistance.
Two identical capacitors in parallel have twice the capaci-
tance and half the ESR as compared to a single capacitor of
the same make. On a similar note, if a large-value, high-ESR
capacitor (tantalum, for example) is to be used as the pri-
mary output capacitor, the net output ESR can be signifi-
cantly reduced by placing a low-ESR ceramic capacitor in
parallel with this primary output capacitor.
Ripple is increased when the LM2788 is gain hopping. Thus,
in the presence of high currents, ripple is likely to vary
significantly over the input voltage, depending on wether or
not the part is gain hopping.
CAPACITORS
The LM2788 requires 4 external capacitors for proper opera-
tion. Surface-mount multi-layer ceramic capacitors are rec-
ommended. These capacitors are small, inexpensive and
have very low equivalent series resistance (ESR, ≤15mΩ
typ.). Tantalum capacitors, OS-CON capacitors, and alumi-
num electrolytic capacitors generally are not recommended
for use with the LM2788 due to their high ESR, as compared
to ceramic capacitors.
For most applications, ceramic capacitors with X7R or X5R
temperature characteristic are preferred for use with the
LM2788. These capacitors have tight capacitance tolerance
(as good as
and typically have little voltage coefficient.
Capacitors with Y5V and/or Z5U temperature characteristic
are generally not recommended for use with the LM2788.
These types of capacitors typically have wide capacitance
tolerance (+80%, -20%), vary significantly over temperature
(Y5V: +22%, -82% over -30
-56% over +10
coefficients. Under some conditions, a nominal 1µF Y5V or
Z5U capacitor could have a capacitance of only 0.1µF. Such
detrimental deviation is likely to cause these Y5V and Z5U of
capacitors to fail to meet the minimum capacitance require-
ments of the LM2788.
The table below lists some leading ceramic capacitor manu-
facturers.
OUTPUT CAPACITOR
The output capacitor of the LM2788 plays an important part
in LM2788 performance. In typical high-current applications,
a 10µF low-ESR (ESR = equivalent series resistance) ce-
ramic capacitor is recommended for use. For lighter loads,
the output capacitance may be reduced (capacitance as low
as 1µF for output currents ≤ 60mA is usually acceptable).
The performance of the part should be evaluated with spe-
±
15% over -55
Vishay-Vitramon
Manufacturer
Taiyo-Yuden
Murata
TDK
AVX
±
10%), hold their value over temperature (X7R:
o
o
C to 125
C to +85
o
C; X5R:
o
C range), and have poor voltage
o
C to +85
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Contact Information
www.t-yuden.com
www.murata.com
www.vishay.com
±
www.avx.com
15% over -55
o
C range; Z5U: +22%,
(Continued)
o
C to 85
o
C),
10
cial attention paid to efficiency and output ripple to ensure
the capacitance chosen on the output yields performance
suitable for the application. In extreme cases, excessive
ripple could cause control 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 LM2788 output.
FLYING CAPACITORS
The flying capacitors (C
input to the output, and thus are like the engine of the charge
pump. Low-ESR ceramic capacitors with X7R or X5R tem-
perature 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. 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 LM2788.
The flying capacitance determines the strength of the charge
pump-the larger the capacitance, the bigger the engine. ESR
in the flying capacitors negatively affects the strength of the
charge pump and should be minimized, as ESR contributes
to undesired output resistance. If capacitors are too small
the LM2788 could spend excessive amount of time gain
hopping: decreasing efficiency, increasing output voltage
ripple, and possibly impeding the ability of the part to regu-
late. On the other hand, if the flying capacitors are too large
they could potentially overwhelm the output capacitor, result-
ing in increased output voltage ripple.
INPUT CAPACITOR
If the flying capacitors are the charge pump engine, the input
capacitor (CIN) is the fuel tank: a reservoir of charge that
aids a quick transfer of charge from the supply to the flying
capacitors during the charge phase of operation. The input
capacitor helps to keep the input voltage from drooping at
the start of the charge phase, when the flying capacitor is
first connected to the input, and helps to filter noise on the
input pin that could adversely affect sensitive internal analog
circuitry biased off the input line. As mentioned above, an
X7R/X5R ceramic capacitor is recommended for use. As a
general recommendation, the input capacitor should be cho-
sen to match the output capacitor.
POWER DISSIPATION
LM2788 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. When operating within specified operating ratings,
the peak power dissipation (PD) of all LM2788 voltage op-
tions occurs with the LM2788-1.5 operating at the maximum
rated operating output current of 120mA. With an input volt-
age of 5.5V, the power efficiency (E) of the LM2788-1.5
bottoms out at 54%. Assuming a typical junction-to-ambient
thermal resistance (θJA) for the MSOP package of 220˚C/
Watt, the junction temperature (T
below for a part operating at the maximum rated ambient
temperature (T
A
) of 85˚C.
1
and C
2
) transfer charge from the
J
) of the part is calculated