A8287 ALLEGRO [Allegro MicroSystems], A8287 Datasheet - Page 11

• Manufacturer Part Number: A8287
• Description: LNB Supply and Control Voltage Regulator
• Manufacturer: ALLEGRO [Allegro MicroSystems]

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 (T
2. Defi ne the maximum running junction temperature (T
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:
where:
(b) Estimate the peak current in boost stage I
(c) Estimate boost R
tion temperature. R
Status (I
∆V
V
Bit
D
0
1
2
3
4
5
6
7
is the voltage drop of the boost diode, and
REG
can be taken from the specifi cation table.
2
Name
I
C Read) Register Table
PK
OCP
PNG
VUV
TSD
DIS
D = 1 – [V
= V
DSBOOST
OUT
DS
Thermal Shutdown
Overcurrent
Reserved
Reserved
Power Not Good
LNB output disabled
Reserved
V
×
(R
IN
IN
[ I
Undervoltage
DSBOOST
/ (V
is a function of junction tempera-
LOAD
OUT
/ (0.89
+ V
) at maximum running junc-
Function
D
×
+ ∆V
V
IN
)]
REG
PK
)]
:
A
).
LNB Supply and Control Voltage Regulator
www.allegromicro.com
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 [phone]
J
)of
ture and it rises by 2.7 mΩ/ºC with respect to the specifi ed
fi gure, R
(d) Determine losses in each block P
value of V
(i) When V
sipation occurs at minimum input voltage.
where
and Pd_sw (switching losses estimate); worst case = 70 mW.
(ii) When V
dissipation in this case occurs at maximum input voltage.
where:
Step 4. Determine the thermal impedance required in the
solution:
The R
the R
Note: For maximum effectiveness, the PCB area underneath
the IC should be fi lled 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.
Pd_control = 15 mA
Pd_control = 15 mA
ØJA
ØJA
Actual R
Pd_Rds = I
vs. Area charts on the following page.
Pd_lin = ∆V
Pd_lin = (V
DSBOOST(25ºC)
for one or two layer PCBs can be estimated from
IN
P
IN
R
P
TOT
, perform either (i) or (ii):
IN
TOT
ØJA
< V
> V
DSBOOST
= Pd_control + Pd_lin
= (T
= Pd_Rds + Pd_sw + Pd_control + Pd_lin
OUT
OUT
2
, when T
PK
IN
J
REG
– T
+ V
= R
×
– V
+ V
×
×
A
×
R
DSBOOST(25ºC)
) / P
D
D
DSBOOST
I
D
V
+ ∆V
V
LOAD
– V
+ ∆V
IN
IN
j
TOT
equals 25ºC.
OUT
REG
×
A8285/A8287
)
REG
×
D
. Note that worst case dis-
+ [(T
I
. Note that worst case
LOAD
TOT
j
; based on the relative
– 25)
×
2.7 mΩ]
11