參數(shù)資料
型號(hào)： LM2770
廠商： National Semiconductor Corporation
英文描述： High Efficiency Switched Capacitor Step-Down DC/DC Regulator with Sleep Mode
中文描述： 高效率開關(guān)電容降壓DC / DC變換器與睡眠模式穩(wěn)壓器
文件頁(yè)數(shù)： 11/14頁(yè)
文件大小： 796K
代理商： LM2770
Operation Description
(Continued)
source. This can be done, but the voltage on the output pin
of the LM2770 must not be brought above the input voltage.
The output pin will draw a small amount of current when
driven externally due the internal feedback resistor divider
connected between V
OUT
and GND.
SOFT START
The LM2770 employs soft start circuitry to prevent excessive
input inrush currents during startup. At startup, the output
voltage gradually rises from 0V to the nominal output volt-
age. This occurs in 200μs (typ.). Soft-start is engaged when
the part is enabled, including situations where voltage is
established simultaneously on the V
IN
and EN pins.
THERMAL SHUTDOWN
Protection from overheating-related damage is achieved
with a thermal shutdown feature. When the junction tem-
perature rises to 150
o
C (typ.), the part switches into shut-
down mode. The LM2770 disengages thermal shutdown
when the junction temperature of the part is reduced to
140
o
C (typ.). Due to the high efficiency of the LM2770,
thermal shutdown and/or thermal cycling should not be en-
countered when the part is operated within specified input
voltage, output current, and ambient temperature operating
ratings. If thermal cycling is seen under these conditions, the
most likely cause is an inadequate PCB layout that does not
allow heat to be sufficiently dissipated out of the LLP pack-
age.
SHORT-CIRCUIT AND CURRENT LIMIT PROTECTION
The LM2770 charge pump contains circuitry that protects the
device from destructive failure in the event of a direct short to
ground on the output. This short-circuit protection circuit
limits the output current to 400mA (typ.) when the output
voltage is below 165mV (typ.). The sleep-mode LDO con-
tains a 60mA (typ.) current limit circuit.
Application Information
RECOMMENDED CAPACITOR TYPES
The LM2770 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,
≤
15m
typ.). Tantalum capacitors, OS-CON capacitors, and alumi-
num electrolytic capacitors generally are not recommended
for use with the LM2770 due to their high ESR, as compared
to ceramic capacitors.
For most applications, ceramic capacitors with an X7R or
X5R temperature characteristic are preferred for use with the
LM2770. These capacitors have tight capacitance tolerance
(as good as
±
10%) and hold their value over temperature
(X7R:
±
15% over -55
o
C to 125
o
C; X5R:
±
15% over -55
o
C to
85
o
C).
Capacitors with a Y5V or Z5U temperature characteristic are
generally not recommended for use with the LM2770. These
types of capacitors typically have wide capacitance toler-
ance (+80%, -20%) and vary significantly over temperature
(Y5V: +22%, -82% over -30
o
C to +85
o
C range; Z5U: +22%,
-56% over +10
o
C to +85
o
C range). Under some conditions, a
1μF-rated Y5V or Z5U capacitor could have a capacitance
as low as 0.1μF. Such detrimental deviation is likely to cause
Y5V and Z5U capacitors to fail to meet the minimum capaci-
tance requirements of the LM2770.
Net capacitance of a ceramic capacitor decreases with in-
creased DC bias. This degradation can result in lower ca-
pacitance than expected on the input and/or output, resulting
in higher ripple voltages and currents. Using capacitors at
DC bias voltages significantly below the capacitor voltage
rating will usually minimize DC bias effects. Consult capaci-
tor manufacturers for information on capacitor DC bias char-
acteristics.
Capacitance characteristics can vary quite dramatically with
different application conditions, capacitor types, and capaci-
tor manufacturers. It is strongly recommended that the
LM2770 circuit be thoroughly evaluated early in the
design-in process with the mass-production capacitors of
choice. This will help to ensure that any such variability in
capacitance does not negatively impact circuit performance.
The table below lists some leading ceramic capacitor manu-
facturers.
Manufacturer
AVX
Murata
Taiyo-Yuden
TDK
Vishay-Vitramon
Contact Information
www.avx.com
www.murata.com
www.t-yuden.com
www.component.tdk.com
www.vishay.com
OUTPUT CAPACITOR AND OUTPUT VOTAGE RIPPLE
The output capacitor in the LM2770 circuit (C
) directly
impacts the magnitude of output voltage ripple. Other promi-
nent factors also affecting output voltage ripple include input
voltage, output current and flying capacitance. Due to the
complexity of multi-gain and PFM switching, providing equa-
tions or models to approximate the magnitude of the ripple
can not be easily accomplished. But one important generali-
zation can be made: increasing (decreasing) the output ca-
pacitance will result in a proportional decrease (increase) in
output voltage ripple. This can be observed in the output
voltage ripple waveforms in the
Typical Performance Char-
acteristics
section.
In typical high-current applications, a 10μF low-ESR ceramic
output capacitor is recommended. Different output capaci-
tance values can be used to reduce ripple, shrink the solu-
tion size, and/or cut the cost of the solution. But changing the
output capacitor may also require changing the flying capaci-
tors and/or input capacitor to maintain good overall circuit
performance. Performance of the LM2770 with different ca-
pacitor setups in discussed in the section
Recommended
Capacitor Configurations
.
High ESR in the output capacitor increases output voltage
ripple. If a ceramic capacitor is used at the output, this is
usually not a concern because the ESR of a ceramic capaci-
tor is typically vey low and has only a minimal impact on
ripple magnitudes. If a different capacitor type with higher
ESR is used (tantalum, for example), the ESR could result in
high ripple. To eliminate this effect, the net output ESR can
be significantly reduced by placing a low-ESR ceramic ca-
pacitor in parallel with the primary output capacitor. The low
ESR of the ceramic capacitor will be in parallel with the
higher ESR, resulting in a low net ESR based on the prin-
ciples of parallel resistance reduction.
Due to the PFM nature of the LM2770, output voltage ripple
is highest at light loads. To eliminate this ripple, consider
running the LM2770 in sleep mode when load currents are
20mA or less. Sleep mode disables the charge pump and
enables the internal low-noise bypass linear regulator (LDO).
L
www.national.com
11
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