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參數(shù)資料
| 型號： | LT1054MJ8#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHED CAPACITOR REGULATOR, 40 kHz SWITCHING FREQ-MAX, CDIP8 |
| 封裝： | 0.300 INCH, LEAD FREE, HERMETIC SEALED, CERAMIC, DIP-8 |
| 文件頁數(shù)： | 16/16頁 |
| 文件大?。?/td> | 264K |
| 代理商： | LT1054MJ8#PBF |
LT1054/LT1054L
9
1954lfg
APPLICATIONS INFORMATION
wheredV=peak-to-peakrippleandf=oscillatorfrequency.
For output capacitors with significant ESR a second term
must be added to account for the voltage step at the switch
transitions. This step is approximately equal to:
(2IOUT)(ESR of COUT)
Power Dissipation
The power dissipation of any LT1054 circuit must be
limited such that the junction temperature of the device
does not exceed the maximum junction temperature rat-
ings. The total power dissipation must be calculated from
two components, the power loss due to voltage drops
in the switches and the power loss due to drive current
losses. The total power dissipated by the LT1054 can be
calculated from:
P ≈ (VIN – |VOUT|)(IOUT) + (VIN)(IOUT)(0.2)
where both VIN and VOUT are referred to the ground pin
(Pin3) of the LT1054. For LT1054 regulator circuits, the
power dissipation will be equivalent to that of a linear
regulator. Due to the limited power handling capability of
the LT1054 packages, the user will have to limit output
currentrequirementsortakestepstodissipatesomepower
external to the LT1054 for large input/output differentials.
This can be accomplished by placing a resistor in series
with CIN as shown in Figure 6. A portion of the input
voltage will then be dropped across this resistor without
affecting the output regulation. Because switch current is
approximately2.2timestheoutputcurrentandtheresistor
will cause a voltage drop when CIN is both charging and
discharging, the resistor should be chosen as:
RX = VX/(4.4 IOUT)
where:
VX ≈ VIN – [(LT1054 Voltage Loss)(1.3) + |VOUT|]
and IOUT = maximum required output current. The factor
of 1.3 will allow some operating margin for the LT1054.
For example: assume a 12V to –5V converter at 100mA
outputcurrent.Firstcalculatethepowerdissipationwithout
an external resistor:
P = (12V –
|–5V|)(100mA) + (12V)(100mA)(0.2)
P = 700mW + 240mW = 940mW
At
θJA of 130°C/W for a commercial plastic device this
would cause a junction temperature rise of 122°C so that
the device would exceed the maximum junction tempera-
ture at an ambient temperature of 25°C. Now calculate the
power dissipation with an external resistor (RX). First find
how much voltage can be dropped across RX. The maxi-
mum voltage loss of the LT1054 in the standard regulator
configuration at 100mA output current is 1.6V, so:
VX = 12V – [(1.6V)(1.3) + |–5V|] = 4.9V and
RX = 4.9V/(4.4)(100mA) = 11Ω
This resistor will reduce the power dissipated by the
LT1054 by (4.9V)(100mA) = 490mW. The total power dis-
sipated by the LT1054 would then be (940mW – 490mW)
= 450mW. The junction temperature rise would now be
only 58°C. Although commercial devices are guaranteed
to be functional up to a junction temperature of 125°C, the
specifications are only guaranteed up to a junction tem-
perature of 100°C, so ideally you should limit the junction
temperature to 100°C. For the above example this would
mean limiting the ambient temperature to 42°C. Other
steps can be taken to allow higher ambient temperatures.
The thermal resistance numbers for the LT1054 packages
represent worst-case numbers with no heat sinking and
still air. Small clip-on type heat sinks can be used to lower
the thermal resistance of the LT1054 package. In some
systems there may be some available airflow which will
help to lower the thermal resistance. Wide PC board traces
from the LT1054 leads can also help to remove heat from
the device. This is especially true for plastic packages.
C1
R2
CIN
COUT
VOUT
LT1054 F06
VIN
R1
RX
LT1054
FB/SHDN
CAP+
GND
CAP–
V+
OSC
VREF
VOUT
+
Figure 6
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