LTC3785 LINER [Linear Technology], LTC3785 Datasheet - Page 22

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

LTC3789
applicaTions inForMaTion
Design Example
V
V
I
f = 400kHz
Maximum ambient temperature = 60°C
Set the frequency at 400kHz by applying 1.2V on the FREQ
pin (see Figure 7). The 10µA current flowing out of the
FREQ pin will give 1.2V across a 120k resistor to GND. The
inductance value is chosen first based on a 30% ripple cur-
rent assumption. In the buck region, the ripple current is:
The highest value of ripple current occurs at the maximum
input voltage. In the boost region, the ripple current is:
The highest value of ripple current occurs at V
V
A 6.8µH inductor will produce 11% ripple in the boost region
(V
The R
maximum current sense voltage specification with some
accommodation for tolerances.
Select an R
22
R
OUT(MAX)
IN
OUT
OUT
SENSE
IN
I
∆I
I
∆I
RIPPLE,BUCK
RIPPLE,BOOST
= 5V to 18V
= 6V) and 29% ripple in the buck region (V
/2.
L,BUCK
L,BOOST
= 12V
SENSE
=
= 5A
2 • I
SENSE
resistor value can be calculated by using the
=
OUT(MAX,BOOST)
=
V
f • L
OUT
=
f • L
=
of 10mΩ.
V
IN
∆I
∆I
• 1 –
•
L,BOOST

 
L,BUCK
2 • 140mV • V



I
1 –
OUT
I
IN
V
V
OUT
• V
V
IN
• 100
V
OUT
• 100
OUT
IN

 



+ ∆I
%
IN(MIN)
%
L,BOOST
IN
• V
= 18V).
IN(MIN)
IN
=
Output voltage is 12V. Select R1 as 20k. R2 is:
Select R2 as 280k. Both R1 and R2 should have a toler-
ance of no more than 1%.
Selecting MOSFET Switches
The MOSFETs are selected based on voltage rating and
R
fied for operation with the available gate voltage amplitude.
In this case, the amplitude is 5.5V and MOSFETs with an
R
Select QA and QB. With 18V maximum input voltage
MOSFETs with a rating of at least 30V are used. As we do
not yet know the actual thermal resistance (circuit board
design and airflow have a major impact) we assume that
the MOSFET thermal resistance from junction to ambient
is 50°C/W.
If we design for a maximum junction temperature, T
= 125°C, the maximum R
First, calculate the maximum power dissipation:
The maximum dissipation in QA occurs at minimum input
voltage when the circuit operates in the boost region and
QA is on continuously. The input current is then:
We calculate a maximum value for R
DS(ON)
DS(ON)
R2 =
P
P
R
R
V
D(MAX)
D(MAX)
DS(ON)
DS(ON)
OUT
V
value. It is important to ensure that the part is speci-
value specified at V
V
IN(MIN)
• I
OUT
OUT(MAX)
(125°C) <
(125°C) <
0.8
=
=
(125 − 60)



• R1
T
J(MAX)
50
– R1
, OR 12A
R
(j− a)
(12A)
− T
I
IN(MAX)
1.3W
P
= 1.3W
DS(ON)
D(MAX)
A(MAX)
GS
2
= 4.5V can be used.
= 0.009Ω
2
value can be calculated.



DS(ON)
:
J(MAX)
3789f