A8287 Просмотр технического описания (PDF) - Allegro MicroSystems
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A8287 Datasheet PDF : 17 Pages
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A8285/A8287
LNB Supply and Control Voltage Regulator
Power Dissipation
To ensure that the device operates within the safe operating
temperature range, several checks should be performed. An
approximate operating junction temperature can be deter-
mined by estimating the power losses and the thermal imped-
ance characteristics of the printed circuit board solution. To
do so, perform the following procedure:
1. Estimate the maximum ambient temperature (TA).
2. Define the maximum running junction temperature (TJ)of
A8285/A8287. Note that the absolute maximum junction
temperature should never exceed 150ºC.
3. Determine worst case power dissipation:
(a) Estimate the duty cycle D:
D = 1 – [VIN / (VOUT + VD + ΔVREG)]
where:
VD is the voltage drop of the boost diode, and
ΔVREG can be taken from the specification table.
(b) Estimate the peak current in boost stage IPK:
× × IPK = VOUT [ ILOAD / (0.89 VIN)]
(c) Estimate boost RDS (RDSBOOST ) at maximum running junc-
tion temperature. RDSBOOST is a function of junction tempera-
ture and it rises by 2.7 mΩ/ºC with respect to the specified
figure, R , DSBOOST(25ºC) when Tj equals 25ºC.
× Actual RDSBOOST = RDSBOOST(25ºC) + [(Tj – 25) 2.7 mΩ]
(d) Determine losses in each block PTOT; based on the relative
value of VIN, perform either (i) or (ii):
(i) When VIN < VOUT + VD + ΔVREG. Note that worst case dis-
sipation occurs at minimum input voltage.
where
PTOT = Pd_Rds + Pd_sw + Pd_control + Pd_lin
× × Pd_Rds = I2PK RDSBOOST D
× Pd_control = 15 mA VIN
× Pd_lin = ΔVREG ILOAD
and Pd_sw (switching losses estimate); worst case = 70 mW.
(ii) When VIN > VOUT + VD + ΔVREG. Note that worst case
dissipation in this case occurs at maximum input voltage.
PTOT = Pd_control + Pd_lin
where:
× Pd_control = 15 mA VIN
× Pd_lin = (VIN – VD – VOUT ) ILOAD
Status (I2C Read) Register Table
Bit Name
Function
0
TSD Thermal Shutdown
1
OCP Overcurrent
2
Reserved
3
Reserved
4
PNG Power Not Good
5
DIS LNB output disabled
6
Reserved
7
VUV VIN Undervoltage
Step 4. Determine the thermal impedance required in the
solution:
RØJA = (TJ – TA) / PTOT
The RØJA for one or two layer PCBs can be estimated from
the RØJA vs. Area charts on the following page.
Note: For maximum effectiveness, the PCB area underneath
the IC should be filled copper and connected to pins 4 and
13 for A8285, and pins 6, 7, 18, and 19 for A8287. Where
a PCB with two or more layers is used, apply thermal vias,
placing them adjacent to each of the above pins, and under-
neath the IC.
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