Datasheet

LTC3789
16
3789fa
applicaTions inForMaTion
The Typical Application on the first page is a basic
LTC3789 application circuit. External component selec-
tion is driven by the load requirement, and begins with
the selection of R
SENSE
and the inductor value. Next, the
power MOSFETs are selected. Finally, C
IN
and C
OUT
are
selected. This circuit can be configured for operation up
to an input voltage of 38V.
R
SENSE
Selection and Maximum Output Current
R
SENSE
is chosen based on the required output current.
The current comparator threshold sets the peak of the
inductor current in the boost region and the maximum
inductor valley current in the buck region. In the boost
region, the maximum average load current at V
IN(MIN)
is:
I
OUT(MAX,BOOST)
=
140mV
R
SENSE
–
∆I
L
2






•
V
IN(MIN)
V
OUT
where ∆I
L
is peak-to-peak inductor ripple current. In the
buck region, the maximum average load current is:
I
OUT(MAX,BUCK)
=
90mV
R
SENSE
+
∆I
L
2
Figure 6 shows how I
LOAD(MAX)
• R
SENSE
varies with in-
put and output voltage.
The maximum current sensing R
SENSE
value for the boost
region is:
R
SENSE(MAX)
=
2 • 140mV • V
IN(MIN)
2 • I
OUT(MAX,BOOST)
• V
OUT
+ ∆I
L,BOOST
• V
IN(MIN)
The maximum current sensing R
SENSE
value for the buck
region is:
R
SENSE(MAX)
=
2 • 90mV
2 • I
OUT(MAX,BUCK)
– ∆I
L,BUCK
The final R
SENSE
value should be lower than the calculated
R
SENSE(MAX)
in both the boost and buck regions. A 20%
to 30% margin is usually recommended.
Programming Input/Output Current Limit
As shown in Figures 7 and 8, input/output current sense
resistor R
SENSE2
should be placed between the bulk capaci-
tor for V
IN
/V
OUT
and the decoupling capacitor. A lowpass
filter formed by R
F
and C
F
is recommended to reduce the
switching noise and stabilize the current loop. The input/
output current limit is set by the I
LIM
pin for 50mV, 100mV
or 140mV with I
LIM
pulled to the GND, floating, or tied to
INTV
CC
, respectively. If input/output current limit is not
desired, the I
OSENSE
+
and I
OSENSE
–
pins should be shorted
to either V
OUT
or V
IN
.
Figure 6. Load Current vs V
IN
/V
OUT
V
IN
/V
OUT
(V)
0.1
90
100
I
LOAD(MAX)
• R
SENSE
(mV)
110
120
130
140
160
1 10
3789 F06
150
Figure 7. Programming Output Current Limit
Figure 8. Programming Input Current Limit
LTC3789
FROM
CONTROLLER
V
OUT
TO
SYSTEM
V
OUT
I
OSENSE
–
R
SENSE2
I
OSENSE
+
R
F
100Ω
R
F
100Ω
C
F
3789 F07
12
+
LTC3789
FROM DC
POWER INPUT
TO DRAIN OF
SWITCH A
I
OSENSE
–
R
SENSE2
I
OSENSE
+
R
F
100Ω
R
F
100Ω
C
F
3789 F08
12
+