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参数资料
| 型号： | LTC3872ETS8#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 650 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | 1 MM HEIGHT, LEAD FREE, PLASTIC, MO-193, TSOT-23, 8 PIN |
| 文件页数： | 5/22页 |
| 文件大小： | 245K |
| 代理商： | LTC3872ETS8#PBF |
LTC3872
13
3872fb
both available in surface mount packages. In the case of
tantalum, it is critical that the capacitors have been surge
tested for use in switching power supplies. An excellent
choice is AVX TPS series of surface mount tantalum. Also,
ceramic capacitors are now available with extremely low
ESR, ESL and high ripple current ratings.
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 6b). The input voltage source
impedance determines the size of the input capacitor,
which is typically in the range of 10F to 100F. 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:
IRMS(CIN) = 0.3
VIN(MIN)
L f
DMAX
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!
Efficiency Considerations: How Much Does VDS
Sensing Help?
Theefficiencyofaswitchingregulatorisequaltotheoutput
power divided by the input power (
×100%).
Percent efficiency can be expressed as:
% Efficiency = 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
individuallossestodeterminewhatislimitingtheefficiency
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 250A 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 efficiency that results from not
having a sense resistor. The losses in the power MOSFET
are equal to:
PFET =
IO(MAX)
1– DMAX
2
RDS(ON) DMAX ρT
+ k VO1.85
IO(MAX)
1– DMAX
CRSS f
The I2R power savings that result from not having a
discretesenseresistorcanbecalculatedalmostbyinspec-
tion.
PR(SENSE) =
IO(MAX)
1– DMAX
2
RSENSE DMAX
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
thefrontpage.Themaximumloadcurrentis7A(10Apeak)
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
applicaTions inForMaTion
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