CS5112
Circuit Description: continued
V
IN
V
LIN
V
OUT
I
BIAS
R
BIAS
64.9kW
C
OSC
Oscillator
ENABLE
+
COMP
V
SW
Logic
Current Sense Amplifier
Base Drive
+
R
S
-
Gnd
1.4A
C
OUT
COMP
Switcher
Error
Amplifier
-
+
Figure 6: Block diagram of the 1.4A current mode control switching regulator portion of the CS5112 in a boost configuration.
Application Notes
Design Procedure for Boost Topology
This section outlines a procedure for designing a boost switch-
ing power supply operating in the discontinuous mode.
Step 1
Determine the output power required by the load.
P
OUT
= I
OUT
V
OUT
(1)
Step 2
Choose C
OSC
based on the target oscillator frequency with an
external resistor value, R
BIAS
= 64.9k�½. (See Figure 1d).
V
SW
V
OUT
V
IN
V
SAT
0
I
SW
I
Peak
t
0
I
D
I
Peak
t
0
t
Figure 7: Voltage and current waveforms for boost topology in CS5112.
-
Switcher Shutdown
1.25V
Bandgap
Reference
R
1
R
2
R
3
V
FB1
Multiplexer
V
FB2
SELECT
Step 3
Next select the output voltage feedback sense resistor
divider as follows (Figure 8).
For V
FB1
active, choose a value for R
1
and then solve for
R
EQ
where:
R
EQ
=
R
1
V
OUT
-1
V
FB1
.
(3a)
V
OUT
R
1
For V
FB2
active, find:
V
FB1
= V
OUT
(
R
EQ
R
1
+ R
EQ
)
R
EQ
, (3b)
{
V
FB1
V
R2
R
2
V
FB2
R
3
and then calculate R
2
where:
V
R2
I
R2
V
FB1
- V
FB2
V
FB1
/R
EQ
. (3c)
Figure 8. Feedback sense
resistor divider connected
between V
OUT
and ground.
R
2
=
=
Then find R
3
, where:
R
3
= R
EQ
- R
2
.
(3d)
7
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
