Circuit Description: continued
V
IN
V
LIN
V
OUT
V
SW
C
OUT
COMP
I
BIAS
Logic
Base Drive
1.4A
R
BIAS
64.9kΩ
Current Sense Amplifier
R
S
-
C
OSC
Gnd
Oscillator
ENABLE
-
Switcher Shutdown
1.25V
V
REG
Bandgap
Reference
Over Voltage
R
1
V
Switcher
Error
Amplifier
FB1
COMP
-
Multiplexer
R
2
V
FB2
R
3
SELECT
Figure 6: Block diagram of the 1.4A current mode control switching regulator portion of the CS5111 in a boost configuration.
Application Notes
Step 3
Design Procedure for Boost Topology
Next select the output voltage feedback sense resistor
divider as follows (Figure 8).
This section outlines a procedure for designing a boost switch-
ing power supply operating in the discontinuous mode.
For VFB1 active, choose a value for R1 and then solve for
REQ where:
Step 1
Determine the output power required by the load.
R1
VOUT
VFB1
V
P
OUT = IOUTVOUT
(1)
OUT
.
REQ
=
(3a)
-1
Step 2
R
1
V
Choose COSC based on the target oscillator frequency with an
external resistor value, RBIAS = 64.9kΩ. (See Figure 1d).
FB1
For VFB2 active, find:
V
R
R2
2
R
V
EQ
FB2
V
SW
REQ
R + R
R
3
V
FB1 = VOUT
{
, (3b)
V
)
OUT
(
1
EQ
Figure 8. Feedback sense
resistor divider connected
between VOUT and ground.
V
IN
and then calculate R2 where:
V
SAT
VFB1 VFB2
-
VFB1/REQ
VR2
IR2
t
0
. (3c)
R2 =
=
I
SW
I
Peak
Then find R3, where:
R3 = REQ - R2.
t
0
(3d)
I
D
I
Peak
0
t
Figure 7: Voltage and current waveforms for boost topology in CS5111.
7
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
