AT9933
Design Example Circuit
D
2
(optional)
L
1
R
D
VDC
Q
1
C
D
D
1
R
CS2
R
CS1
R
S2A
C
2
R
S1
VIN
GATE
CS1
R
REF1
GND
VDD
PWMD
CS2
R
REF2
REF
C
3
R
S2B
D3
C
1
L
2
-
C
O
VO
+
AT9933
Design Example
The choice of the resistor dividers to set the input and output
current levels is illustrated by means of the design example
given below.
The parameters of the power circuit are:
V
IN MIN
= 9V
V
IN MAX
= 16V
V
O
= 28V
I
O
= 0.35A
f
S MIN
= 300kHz
Using these parameters, the values of the power stage in-
ductors and capacitor can be computed as (see Application
Note AN-H51 for details):
L1 = 82µH
L2 = 150µH
C1 = 0.22µF
The input and output currents for this design are:
I
IN MAX
= 1.6A
ΔI
IN
= 0.21A
I
O
= 350mA
ΔI
O
= 87.5mA
Current Limits
The current sense resistor (R
CS2
), combined with the other
resistors (R
S2
& R
REF2
), determines the output current limits.
The current sense resistor (R
CS1
), combined with the other
resistors (R
S1
& R
REF1
), determines the input current limits.
The resistors can be chosen using the following equations:
R
S
R
REF
R
S
R
REF
(2)
+ 0.05V
(3)
+ 0.1V
I X R
CS
= 1.2V X
∆I X R
CS
= 0.1V X
Where
I
is the current (either
I
O
or
I
IN
) and
ΔI
is the peak-to-
peak ripple in the current (either
ΔI
O
or
ΔI
IN
).
For the input side, the current level used in the equations
should be larger than the maximum input current so that it
does not interfere with the normal operation of the circuit.
The peak input current can be computed as:
∆ I in
2
(4)
I
IN,PK
= I
IN,MAX
+
= 1.706A
7
SUPERTEX [ Supertex, Inc ]
