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參數(shù)資料
| 型號： | LTC3872ETS8 |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 650 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | 1 MM HEIGHT, PLASTIC, MO-193, TSOT-23, 8 PIN |
| 文件頁數(shù)： | 4/20頁 |
| 文件大小： | 261K |
| 代理商： | LTC3872ETS8 |
LTC3872
12
3872fa
Input Capacitor Selection
The input capacitor of a boost converter is less critical
than the output capacitor, due to the fact that the inductor
is in series with the input and the input current waveform
is continuous (see Figure 5b). The input voltage source
impedance determines the size of the input capacitor,
which is typically in the range of 10μF to 100μF. A low ESR
capacitor is recommended, although it is not as critical as
for the output capacitor.
The RMS input capacitor ripple current for a boost con-
verter is:
I
V
Lf
D
RMS CIN
IN MIN
MAX
()
.
= 03
Please note that the input capacitor can see a very high
surge current when a battery is suddenly connected to
the input of the converter and solid tantalum capacitors
can fail catastrophically under these conditions. Be sure
to specify surge-tested capacitors!
Efciency Considerations: How Much Does VDS
Sensing Help?
The efciency of a switching regulator is equal to the output
power divided by the input power (
×100%).
Percent efciency can be expressed as:
% Efciency = 100% – (L1 + L2 + L3 + …),
where L1, L2, etc. are the individual loss components as a
percentage of the input power. It is often useful to analyze
individual losses to determine what is limiting the efciency
and which change would produce the most improvement.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for the majority
of the losses in LTC3872 application circuits:
1. The supply current into VIN. The VIN current is the
sum of the DC supply current IQ (given in the Electrical
Characteristics) and the MOSFET driver and control cur-
rents. The DC supply current into the VIN pin is typically
about 250μA and represents a small power loss (much
less than 1%) that increases with VIN. The driver current
results from switching the gate capacitance of the power
MOSFET; this current is typically much larger than the
DC current. Each time the MOSFET is switched on and
then off, a packet of gate charge QG is transferred from
VIN to ground. The resulting dQ/dt is a current that must
be supplied to the Input capacitor by an external supply.
If the IC is operating in CCM:
IQ(TOT) ≈ IQ = f QG
PIC = VIN (IQ + f QG)
2. Power MOSFET switching and conduction losses. The
technique of using the voltage drop across the power
MOSFET to close the current feedback loop was chosen
because of the increased efciency that results from not
having a sense resistor. The losses in the power MOSFET
are equal to:
P
I
D
RD
k
FET
OMAX
MAX
DS ON
MAX
T
=
+
()
–
1
2
ρ
–
.
()
V
I
D
Cf
O
OMAX
MAX
RSS
185
1
The I2R power savings that result from not having a
discrete sense resistor can be calculated almost by inspec-
tion.
P
I
D
RD
RSENSE
OMAX
MAX
SENSE
MAX
()
–
=
1
2
To understand the magnitude of the improvement with
this VDS sensing technique, consider the 3.3V input, 5V
output power supply shown in the Typical Application on
the front page. The maximum load current is 7A (10A peak)
and the duty cycle is 39%. Assuming a ripple current of
40%, the peak inductor current is 13.8A and the average
is 11.5A. With a maximum sense voltage of about 140mV,
the sense resistor value would be 10m
Ω, and the power
dissipated in this resistor would be 514mW at maximum
output current. Assuming an efciency of 90%, this
sense resistor power dissipation represents 1.3% of the
overall input power. In other words, for this application,
the use of VDS sensing would increase the efciency by
approximately 1.3%.
For more details regarding the various terms in these
equations, please refer to the section Boost Converter:
APPLICATIO S I FOR ATIO
WU
U
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