rc5037 Fairchild Semiconductor, rc5037 Datasheet - Page 7

• Manufacturer Part Number: rc5037
• Description: Adjustable Switching Regulator Controller
• Manufacturer: Fairchild Semiconductor

PRODUCT SPECIFICATION
charged and discharged through the enabling and disabling
of the fixed current source. The variable current source is
controlled from the error inputs that are received from the
current and voltage feedback signals. The oscillator off-time
is controlled by the amount of current that is available from
Output Voltage Selection
The RC5037 precision reference is trimmed to be 1.5V nom-
inally. When using the RC5037, the system designer has
complete flexibility in choosing the output voltage for one
regulator from 1.5V to 3.6V. This is done by appropriately
selecting the feedback resistors. These could be 0.1% resis-
tors to realize optimum output accuracy. The following
equations determines the output voltage of the regulator:
For example, for 3.3V:
Input Capacitors
The number of input capacitors required for the RC5037 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
1.5V output at 10A, 5V input, and using the Sanyo capaci-
tors specified in Table 1 which have a 2A ripple current rat-
V
V
No. Caps
OUT
OUT
=
=
1.5
1.5
=
CEXT
I
--------------------------------------- -
out
R4
-------------------- -
R4
-------------------- -
*
1
I
DC DC
R5
rating
R5
+
+
R5
R5
–
REF
=
2
g m
out
1.5
/V
in
2.43k
------------------------------ -
. For example, with a
Constant On-Time Oscillator
I
O
2.0k
g m
6
Figure 2. RC5037 Detailed Block Diagram
+
GNDP
2.0k
=
I
V
V
ON
(1)
3.3V
(2)
H
L
the variable current source to charge the external capacitor up
to the high threshold level of the comparator. The on-time is
set by the constant current source that discharges the external
capacitor voltage down to the lower comparator threshold.
ing, we have DC = 1.5/5 = 0.3, and
so that we need 3 input capacitors.
Short Circuit Considerations
The RC5037 uses a current sensing scheme to limit the load
current if an output fault condition occurs. The current sense
resistor carries the peak current of the inductor, which is
greater than the maximum load current due to ripple current
flowing in the inductor. The RC5037 will begin to limit the
output current to the load by reducing the duty cycle of the
top-side MOSFET driver when the voltage across the cur-
rent-sense resistor exceeds the short circuit comparator
threshold voltage (V
age will temporarily go out of regulation. As the voltage
across the sense resistor becomes larger, the duty cycle of the
top-side MOSFET will continue to be reduced until the cur-
rent limit value is reached. At this point, the RC5037 will
continuously deliver the limit current at a reduced output
voltage level. The short circuit comparator threshold voltage
is typically 90mV, with a tolerance of ±10mV. The ripple
current flowing through the inductor in Figure 1 is 0.6A.
Refer to Application Note AM-53 for detailed discussions.
The sense resistor value can be approximated as follows:
R
No. Caps
SENSE
+12V
+5V
=
2
4
3
8
7
5
=
V
--------------- -
VCCA
IFBL
IFBH
VCCP
SDRV
FBSW
I
10*
----------------------------------- -
th,min
PK
0.03 0.3
2
th
1 TF
–
). When this happens the output volt-
–
65-5037-07
2
=
=
2.29
---------------------------------------------
0.6A
+
V
I
th,min
LOAD,MAX
V OUT
1 TF
RC5037
–
7
(3)