ADM666AAN 데이터 시트보기 (PDF) - Analog Devices
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ADM666AAN Datasheet PDF : 9 Pages
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ADM663A/ADM666A
Temperature Proportional Output
The ADM663A contains a VTC output with a positive tempera-
ture coefficient of +2.5 mV/°C. This may be connected to the
summing junction of the error amplifier (VSET) through a resis-
tor resulting in a negative temperature coefficient at the output
of the regulator. This is especially useful in multiplexed LCD
displays to compensate for the inherent negative temperature
coefficient of the LCD threshold. At +25°C the voltage at the
VTC output is typically 0.9 V. The equations for setting both
the output voltage and the tempco are given below. If this func-
tion is not being used, then VTC should be left unconnected.
( ) VOUT
=
V SET
1 +
R2
R1
+
R2
R3
V SET
− VTC
( ) TCV OUT
= –R2
R3
TCV TC
where VSET = +1.3 V, VTC = +0.9 V, TCVTC = +2.5 mV/°C
SENSE
VOUT2
ADM663A
VSET
VTC
VOUT
R2
R3
R1
Figure 8. ADM663A Temperature Proportional Output
APPLICATION HINTS
Input-Output (Dropout Voltage)
A regulator’s minimum input-output differential or dropout
voltage determines the lowest input voltage for a particular out-
put voltage. The ADM663A/ADM666A dropout voltage is 1 V
at its rated output current. For example when used as a fixed
+5 V regulator, the minimum input voltage is +6 V. At lower
output currents (IOUT < 10 mA) on the ADM663A, VOUT1 may
be used as the output driver in order to achieve lower dropout
voltages. In this case the dropout voltage depends on the voltage
drop across the internal FET transistor. This may be calculated
by multiplying the FET’s saturation resistance by the output
current, for example with VIN = 9 V, RSAT = 20 Ω. Therefore,
the dropout voltage for 5 mA is 100 mV. As the current limit
circuitry is referenced to VOUT2, VOUT2 should be connected to
VOUT1. For high current operation VOUT2 should be used alone
and VOUT1 left unconnected.
+6V TO +16V
INPUT
VIN
ADM663A
VSET GND
SENSE
VOUT2
VOUT1
SHDN
+5V
OUTPUT
Figure 9. Low Current, Low Dropout Configuration
Thermal Considerations
The ADM663A/ADM666A can supply up to 100 mA load cur-
rent and can operate with input voltages up to 16.5 V, but the
package power dissipation and hence the die temperature must
be kept within the maximum limits. The package power dissi-
pation is calculated from the product of the voltage differential
across the regulator times the current being supplied to the load.
The power dissipation must be kept within the maximum limits
given in the Absolute Maximum Ratings section.
PD = (VIN–VOUT)(IL)
The die temperature is dependent on both the ambient tempera-
ture and on the power being dissipated by the device. The
ADM663A/ADM666A contains an internal thermal limiting cir-
cuit which will shut down the regulator if the internal die tem-
perature exceeds 125°C. Therefore, care must be taken to
ensure that, under normal operating conditions, the die tem-
perature is kept below the thermal limit.
TJ = TA + PD (θJA)
This may be expressed in terms of power dissipation as follows:
where:
PD = (TJ – TA)/(θJA)
TJ = Die Junction Temperature (°C)
TA = Ambient Temperature (°C)
PD = Power Dissipation (W)
θJA = Junction to Ambient Thermal Resistance (°C/W)
If the device is being operated at the maximum permitted ambi-
ent temperature of 85°C the maximum power dissipation per-
mitted is:
PD (max) = (TJ (max) – TA)/(θJA)
PD (max) = (125 – 85)/(θJA)
= 40/θJA
θJA = 120°C/W for the 8-pin DIP (N-8) package
θJA = 170°C/W for the 8-pin SOIC (R-8) package
Therefore, for a maximum ambient temperature of 85°C
PD (max) = 333 mW for N-8
PD (max) = 235 mW for R-8
At lower ambient temperatures the maximum permitted power
dissipation increases accordingly up to the maximum limits
specified in the absolute maximum specifications.
The thermal impedance (θJA) figures given are measured in still
air conditions and are reduced considerably where fan assisted
cooling is employed. Other techniques for reducing the thermal
impedance include large contact pads on the printed circuit
board and wide traces. The copper will act as a heat exchanger
thereby reducing the effective thermal impedance.
High Power Dissipation Recommendations
Where excessive power dissipation due to high input-output dif-
ferential voltages and or high current conditions exists, the sim-
plest method of reducing the power requirements on the
regulator is to use a series dropper resistor. In this way the ex-
cess power can be dissipated in the external resistor. As an ex-
ample, consider an input voltage of +12 V and an output
voltage requirement of +5 V @ 100 mA with an ambient tem-
perature of +85°C. The package power dissipation under these
conditions is 700 mW which exceeds the maximum ratings. By
using a dropper resistor to drop 4 V, the power dissipation re-
quirement for the regulator is reduced to 300 mW which is
within the maximum specifications for the N-8 package at
–6–
REV. 0
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