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PRODUCT SPECIFICATION
RC5011
5
A
Design Equations
The inductor (L1) and timing capacitor (CX) values must be
tailored to the input voltage range, output voltage, and load
current requirements of the application. The key to the prob-
lem is to select the correct inductor value for a given oscilla-
tor frequency such that the inductor current rises to a peak
value (Imax) sufficient to meet the average load current
drain. The worst-case conditions for calculating its ability to
supply load current are found at the minimum supply volt-
age. Therefore, V
BAT,MIN
is used to calculate the inductor
value. Conversely, the worst-case condition for output volt-
age ripple will occur at V
BAT,MAX
.
The value of the timing capacitor is set according to the
following equation:
F
O
(Hz) = (5 x 10
-6
)/Cx
The output of the oscillator is measured at pin 2 (CX) and
the voltage at the CX pin will be a triangle waveform. By
pulling the VFB pin above 1.25V, the oscillator square wave
output can be measured directly at pin 6 (DRV). Capacitor
selection will depend on the specific application; higher
operating frequencies will reduce the output voltage ripple
and will allow the use of an inductor with a physically
smaller inductor core, but excessively high frequencies will
reduce load driving capability and efficiency.
Maximum on time of the MOSFET is calculated as follows:
Ton = 1/2F
O
+ 0.5
m
S
The 0.5
m
S term is added to represent the MOSFET gate-dis-
charge time, although it is an approximation only and should
be checked for the specific MOSFET used.
The peak inductor current is:
where:
V
BAT
V
F
I
DC
V
DS,ON
= drain-source on voltage of MOSFET
= supply voltage
= diode forward voltage
= dc load current
Inductor value:
Output filter capacitor:
where V
RIPPLE
= Peak output voltage ripple
To reduce system power consumption when the switch-mode
section is not in use, the circuit shown in Figure 2 is recom-
mended. This circuit prevents any load connected to V
OUT
from drawing current out of V
BAT
.
I
MAX
V
O
)
T
on
V
BAT
V
F
V
BAT
V
DS ON
–
–
+
(
[
]
-F
è
2I
DC
=
LX Henries
)
V
----------------–
V
I
MAX
è
T
ON
=
C1
m
F
(
)
T
ON
------------------------------------------------------------
V
I
V
RIPPLE
-------V
I
DC
+
=
Figure 2. Standard Test Schematic with 12V Shutdown Circuit added
RC5011
VBAT
D1
IN5817
R6
110K
5VIN
EN5V
CX
L1
10
μ
H
R7
13.3K
R1
40K
100K
R2
R3
40K
Q1
2N2222
CX
GND
EN12V
8
+12V
7
6
5
VBAT
VOUT
DRV
VFB
1
2
100pF
C1
300
μ
F
EN5V
EN12V
3
4
M1
Si9410
+5V