10
A. Specifications:
B. Calculate R
OUT
:
C. Determine Frequency Setting Capacitor CT:
.
D. Select Inductor Ripple Current (
∆
I
L
):
E. Determine the Inductors:
F. Output Capacitors:
G. Input Capacitor’s RMS Current:
For 40A with a duty cycle (D) of:
H. Current Sense Resistor (R
SENSE
):
I. R
SENSE
Dissipation:
J. R
L
Selection:
K. V
SET
Computation for No Load Voltage = DAC:
Input Voltage: 12V
Output Voltage: VDAC + 15mV
Output Voltage for Calculations:
V
DAC
= 1.8V + 15mV
Droop Voltage: 65mV
Oscillator Frequency: 400kHz (f
SW
)
Output Current: 40A
R
OUT
V
DROOP
I
OUT
----------------------------
65mV
40A
----------------
1.63m
Ω
=
=
=
1000
100
3000
FREQ
UENCY (kHz)
CAPACITOR CT (pF)
100 150 200
250 300
0
50
350 400
450 500
FIGURE A. OSCILLATOR FREQUENCY vs. TIMING CAPACITOR
From curve above, for 400kHz use 120pF.
Or 8A / Channel
∆
I
L
40A
0.4
×
16A
=
=
Choose 40% of IOUT
L
V
IN
V
OUT
–
f
SW
2
------------
∆
I
L
×
----------------------------------
V
OUT
V
IN
-----------------
×
12V
1.8V
–
200kHz
8A
×
-----------------------------------
1.8V
12V
------------
×
=
=
956nH
=
Sonya 1500
µ
F, 4V OS-CON Capacitors
have an ESR < 10m
Ω
Six capacitors < 1.66m
Ω
Total Capacitance = 9mF
Capacitor
ESR
R
OUT
≅
1.63m
Ω
=
0.3
0.2
0.1
0
0
0.1
0.2
0.3
0.4
0.5
DUTY CYCLE (V
O
/ V
IN
)
CURRENT MUL
TIPLIER
FIGURE B. CURRENT MULTIPLIER vs. DUTY CYCL
E
Use the curve of Figure B
D
V
OUT
V
IN
-----------------
1.8V
12V
------------
0.15
=
=
=
The multiplier from Figure B is 0.24.
I
RMS
0.24
=
40A
×
9.6A
=
Pensioned 470
µ
F, 16V Rubdown ZA series capacitors
have a RMS current rating of 1.6A.
Six capacitors were selected.
R
SENSE
V
CS TH
(
)
MIN
I
OUT
2
---------------
I
RIPPLE
2
-------------------------
+
------------------------------------------------
142mV
20A
8A
+
-------------------------
5.07m
Ω
=
=
=
Use a 5m
Ω
resistor
Power
∴
I
P
2
D
×
R
SENSE
×
=
Where: I
P
= 20A + 4A = 24A
432mW
=
(Using half 0f ripple current
I
RMS
I
PEAK
D
=
Power
24A
=
2
0.15
×
5m
Ω
×
Used a 1W resistor
R
L
ni
R
SENSE
×
gm
R
OUT
2
×
×
------------------------------------------
12.5
5m
Ω
×
2.2mS
1.63m
Ω
2
×
×
--------------------------------------------------------
8.7k
Ω
=
=
=
gm
RL
×
2.2mS
8.7k
19.1
=
×
=
=
am Amplifier Gain
ni =
∆
V
COMP
/ V
CS
(from data sheet)
V
OUT
, the no load voltage programed to the DAC voltage
V
S E T
, the voltage set at the COMP pin
1V
8A
5m
Ω
12.5
×
×
2
---------------------------------------------
+
=
VSET
1V
I
RIPPLE
R
SENSE
ni
×
×
2
----------------------------------------------------------------------
+
=
1V
250mV
+
=
1.25V
=
ISL6560/62 Evaluation Board
ISL6560 Supply Design Sequence
The ISL6560 data sheet describes in more detail the following equations. There are several changes from the computations in
the body of the data sheet. First, an operating frequency of 400kHz was chosen. Next, this design sequence shows the method of
setting the initial no load voltage at the DAC setting and offsetting the no load voltage 15mV above the programmed DAC voltage.
