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PRODUCT SPECIFICATION
RC5036
11
Output Voltage Selection
The RC5036 precision reference is trimmed to be 1.5V nom-
inally. When using the RC5036, the system designer has
complete flexibility in choosing the output voltage for each
regulator from 1.5V to 3.6V. This is done by appropriately
selecting the feedback resistors. These could be 0.1% resistors
to realize optimum output accuracy. The following equations
determine the output voltages of the two regulators:
Switching Regulator:
Linear Regulator:
where R6 > 1.5k
and (R5 + R6)
≤
25k
and R8 > 1.5k
and (R7 + R8)
≤
25k
Example:
For 3.3V,
Input Capacitors
The number of input capacitors required for the RC5036 is
dependent on their ripple current rating, which assures their
rated life. The number required may be determined by
where the duty cycle DC = V
out
/V
in
. For example, with a
1.5V output at 10A, 5V input, and using the Sanyo capaci-
tors specified in Table 1 which have a 2A ripple current rat-
ing, we have DC = 1.5/5 = 0.3, and
so that we need 3 input capacitors.
Linear Regulator Design Considerations
Figure 1 shows the application schematic for the RC5036
with an NPN used for the linear regulator.
Careful consideration must be given to the base current of
the power NPN device. The base current to the power NPN
device is limited by:
The RC5036 op-amp output current (50mA)
The internal power dissipation of the RC5036 package
The
β
of the power NPN device.
The internal RC5036 power dissipation is the most severe
limitation for this application. For optimum reliability, we
require that the junction temperature not exceed 130°C; thus
we can calculate the maximum power dissipation allowable
for this 16-lead SOIC package as follows:
If we assume that the ambient temperature T
A
is 70°C and
the thermal resistance of the 16-lead SOIC package is
112°C/W, then the maximum power dissipation for the IC is:
where P
SW
is the internal power dissipation of the switching
regulator and P
LN
is the internal power dissipation of the linear
regulator. I
OL
is the linear regulator op-amp output current.
For V
OUT
= 3.3V nominal, the worst case output will be
determined by the current used.
For example, for a worst case V
OUT
= 3.135V, the maximum
op-amp output current is:
The power NPN transistor must have a minimum
β
of 75 at
I
L
= 3A in order to meet the internal power dissipation limit
of the 16-SOIC package.
V
OUT
1.5
R6
R5
+
R5
×
=
V
OUT
1.5
R8
R7
+
R7
×
=
V
OUT
1.5
R6
R5
+
R5
×
1.5
6.65k
10k
+
10k
×
3.3V
=
=
=
No. Caps
I
------------------------–
DC
I
rating
2
*
=
(2)
No. Caps
0.3
2
–
10*
--------0.03
2
2.29
=
=
P
D
T
-------------------------------
T
A
–
R
Θ
JA
=
P
D
130
70
–
112
0.533W
≤
=
P
D
P
SW
P
LR
+
=
=
35mA
5.25V
×
(
)
12.6V
V
OUT
–
V
BE
–
(
)
I
OL
0.533W
≤
×
+
I
OL
12.6V
35mA
3.135V
5.25V
0.8V
×
–
(
)
–
–
)
0.533W
40mV
≤
=
β
340mA
≥
75
=