LTC3785 LINER [Linear Technology], LTC3785 Datasheet - Page 15

• Manufacturer Part Number: LTC3785
• Description: High Efficiency, Synchronous, 4-Switch Buck-Boost Controller
• Manufacturer: LINER [Linear Technology]

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
power MOSFETs are selected. Finally, C
selected. This circuit can be configured for operation up
to an input voltage of 38V.
R
R
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
where ∆I
buck region, the maximum average load current is:
Figure 6 shows how the load current (I
varies with input and output voltage.
SENSE
SENSE
I
I
OUT(MAX,BOOST)
OUT(MAX,BUCK)
Selection and Maximum Output Current
is chosen based on the required output current.
L
is peak-to-peak inductor ripple current. In the
160
150
140
130
120
110
100
90
0.1
Figure 6. Load Current vs V
SENSE
=
=
R

 
90mV
SENSE
R
140mV
and the inductor value. Next, the
SENSE
V
IN
/V
OUT
1
+
–
(V)
∆I
2
∆I
L
2
L
LOAD(MAX)

 
IN
/V
•
IN
OUT
V
3789 F06
IN(MIN)
V
and C
OUT
10
IN(MIN)
• R
OUT
SENSE
are
is:
)
The maximum current sensing R
region is:
The maximum current sensing R
region is:
The final R
R
to 30% margin is usually recommended.
Programming Input/Output Current Limit
As shown in Figures 7 and 8, current sense resistor R
should be placed between the buck capacitor for V
and the decoupling capacitor. A lowpass filter formed by R
and C
input/output current limit is set by the I
100mV or 140mV with I
tied to INTV
is not desired, the I
shorted to either V
SENSE(MAX)
POWER INPUT
R
R
2 • I
CONTROLLER
SENSE(MAX)
SENSE(MAX)
FROM DC
F
is recommended to reduce the switching noise. The
OUT(MAX,BOOST)
FROM
Figure 7. Programming Output Current Limit
V
Figure 8. Programming Input Current Limit
SENSE
OUT
CC
in both the boost and buck regions. A 20%
+
, respectively. If input/output current limit
=
=
value should be lower than the calculated
2 • 140mV • V
I
OSENSE +
2 • I
OUT
OSENSE
I
OSENSE +
2
R
100
2
OUT(MAX,BUCK)
F
• V
or V
R
100
LIM
LTC3789
F
R
OUT
LTC3789
SENSE2
R
+
IN
SENSE2
pulled to the GND, floating, or
C
and I
2 • 90mV
F
.
C
+ ∆I
F
I
IN(MIN)
OSENSE –
I
SENSE
OSENSE –
SENSE
1
OSENSE
L,BOOST
R
100
1
F
R
100
F
– ∆I
value for the boost
value for the buck
3789 F08
LTC3789
LIM
–
3789 F07
+
L,BUCK
pins should be
• V
pin for 50mV,
IN(MIN)
TO REMAINDER
OF SYSTEM
TO
SYSTEM
V
OUT
IN
15
SENSE2
/V
OUT
3789f
F