LTC1147-3.3 Просмотр технического описания (PDF) - Linear Technology
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LTC1147-3.3 Datasheet PDF : 16 Pages
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LTC1147-3.3
LTC1147-5/LTC1147L
APPLICATIO S I FOR ATIO
Optimum efficiency is obtained by making the ESR equal
to RSENSE. As the ESR is increased up to 2RSENSE, the
efficiency degrades by less than 1%. If the ESR is greater
than 2RSENSE, the voltage ripple on the output capacitor
will prematurely trigger Burst Mode operation, resulting in
disruption of continuous mode and an efficiency hit which
can be several percent.
Manufacturers such as Nichicon and United Chemicon
should be considered for high performance capacitors.
The OS-CON semiconductor dielectric capacitor available
from Sanyo has the lowest ESR/size ratio of any aluminum
electrolytic at a somewhat higher price. Once the ESR
requirement for COUT has been met, the RMS current
rating generally far exceeds the IRIPPLE(P-P) requirement.
In surface mount applications multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirements of the application. Alumi-
num electrolytic and dry tantalum capacitors are both
available in surface mount configurations. In the case of
tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. An excellent choice
is the AVX TPS series of surface mount tantalums, avail-
able in case heights ranging from 2mm to 4mm. For
example, if 200µF/10V is called for in an application
requiring 3mm height, two AVX 100µF/10V (P/N TPSD
107K010) could be used. Consult the manufacturer for
other specific recommendations.
At low supply voltages, a minimum capacitance at COUT is
needed to prevent an abnormal low frequency operating
1000
800
600
400
L = 50µH
RSENSE = 0.02Ω
L = 25µH
RSENSE = 0.02Ω
200
0
0
L = 50µH
RSENSE = 0.05Ω
1
2
3
4
5
(VIN – VOUT) VOLTAGE (V)
LTC1147 • F04
Figure 4. Minimum Value of COUT
10
mode (see Figure 4). When COUT is made too small, the
output ripple at low frequencies will be large enough to trip
the voltage comparator. This causes Burst Mode opera-
tion to be activated when the LTC1147 series would
normally be in continuous operation. The effect is most
pronounced with low values of RSENSE and can be im-
proved by operating at higher frequencies with lower
values of L. The output remains in regulation at all times.
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators
take several cycles to respond to a step in DC (resistive)
load current. When a load step occurs, VOUT shifts by an
amount equal to ∆ILOAD(ESR), where ESR is the effec-
tive series resistance of COUT. ∆ILOAD also begins to
charge or discharge COUT until the regulator loop adapts
to the current change and returns VOUT to its steady
state value. During this recovery time VOUT can be
monitored for overshoot or ringing which would indi-
cate a stability problem. The external components shown
in the Figure 1 circuit will prove adequate compensation
for most applications.
A second, more severe transient is caused by switching
in loads with large (>1µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in par-
allel with COUT, causing a rapid drop in VOUT. No regulator
can deliver enough current to prevent this problem if the
load switch resistance is low and it is driven quickly. The
only solution is to limit the rise time of the switch drive so
that the load rise time is limited to approximately
(25)CLOAD. Thus a 10µF capacitor would require a 250µs
rise time, limiting the charging current to about 200mA.
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percent efficiency can be
expressed as:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
sn1147 1147fds
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