CS5112
Application Notes: continued
V
IN
Pole due to error amplifier
output impedance and C
1
A
f
z
= 1/2pR4C1
f
P
= 1/pR
4
C
2
G
+G
B
error amplifier gain
V
OUT
= 18V, Select > 2V
V
OUT
= 16V, Select < 0.8V
V
IN
NC
NC
ENABLE
V
REG
V
LIN
I
BIAS
Gnd
Gnd
Gnd
Gnd
RESET
C
Delay
WDI
C
OSC
C
OSC
390pF
C
delay
0.1mF
MICROPROCESSOR
C
-20dB/dec
L=33mH
V
SW
(2)
Gnd
Gnd
R
1
100kW
5V
100mF
ESR<8W
R
BIAS
= 64.9kW
Gain (dB)
C
OUT
88mF
CS5112
f
P
= 1/p R
Load
C
OUT
f
CO
0
Gnd
Gnd
R
2
modulator gain + feedback resistor divider attenuation
946W
7.5kW
C
COMP
0.33mF
(1)
V
FB1
V
FB2
SELECT
COMP
R
3
-G
f
z
= 1/2p ESR C
OUT
Figure 10. Bode plot of error amplifier (OTA) gain and modulator gain
added to the feedback resistor divider attenuation.
Figure 11. A typical application diagram with external components con-
figured in a boost topology.
A pole at point C:
f
p
= 1/(¹R
4
C
2
),
(8g)
Step 9
Finally the watchdog timer period and Power on Reset
time is determined by:
t
Delay
= 1.353
´
C
Delay
R
BIAS
.
(9)
offsets the zero set by the ESR of the output capacitors.
An alternative scheme uses a single capacitor as shown in
Figure 11, to roll the gain off at a relatively low frequency.
Linear Regulator Output Current vs. Input Voltage
100
100
75
75
I
LIN
(mA)
I
LIN
(mA)
50
Q
JA
= 55°C/W
V
IN
= 14V
Max Total Power = 1.18W
50
Q
JA
= 35°C/W
V
IN
= 14V
Max Total Power = 1.86W
25
25
0
0
5
10
15
20
25
30
0
0
5
10
15
20
25
30
V
REG
(V)
V
REG
(V)
Figure 12: The shaded area shows the safe operating area of the CS5112 as a function of I
LIN
, V
REG
, and
Q
JA
. Refer to the table below for typical
loads and voltages.
V
REG
(V)
V
IN
(V)
I
LIN
(mA)
Linear Power
Dissipation
(W)
Worst Case Switcher
Power Available
(Q
JA
= 55¡C/W)
(W)
Worst Case Switcher
Power Available
(Q
JA
= 35¡C/W)
(W)
20
20
20
20
25
25
25
25
14
14
14
14
14
14
14
14
25
50
75
100
25
50
75
100
0.44
0.83
1.22
1.60
0.60
1.11
1.62
2.14
9
0.74
0.35
*
*
0.58
0.07
*
*
1.42
1.03
0.64
0.26
1.26
0.75
0.24
*
* Subjecting the CS5112 to these conditions will exceed the maximum total power that the part can handle, thereby forcing it into thermal limit.
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
