LTC3789 Datasheet, PDF (22/28 Pages) Linear Technology – High Efficiency, Synchronous, 4-Switch Buck-Boost Controller

LTC3789
APPLICATIONS INFORMATION
Design Example
V_IN = 5V to 18V
V_OUT = 12V
I_OUT(MAX) = 5A
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:
∆I_L,BUCK
=
V_OUT
f •L
•


1
–
V_OUT 
V_IN 
I_RIPPLE,BUCK
=
∆I_L,BUCK • 100 _%
I_OUT
The highest value of ripple current occurs at the maximum
input voltage. In the boost region, the ripple current is:
∆I_L,BOOST =
V_{IN •}
f •L

 ^{1 –}
V_IN ^
V_OUT 
I_RIPPLE,BOOST
=
∆I _L,BOOST
I _IN
•
100
%
The highest value of ripple current occurs at V_IN =
V_OUT/2.
A 6.8µH inductor will produce 11% ripple in the boost region
(V_IN = 6V) and 29% ripple in the buck region (V_IN = 18V).
The R_SENSE resistor value can be calculated by using the
maximum current sense voltage specification with some
accommodation for tolerances.
R_SENSE
=
2
2 • 140mV •
• I_{OUT(MAX,BOOST)} • V_OUT
V_IN(MIN)
+ ∆I_L,BOOST
•
V_IN(MIN)
Select an R_SENSE of 10mΩ.
Output voltage is 12V. Select R1 as 20k. R2 is:
R2 = ^{VOUT • R1} – R1
0.8
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_DS(ON) value. It is important to ensure that the part is speci-
fied for operation with the available gate voltage amplitude.
In this case, the amplitude is 5.5V and MOSFETs with an
R_DS(ON) value specified at V_GS = 4.5V can be used.
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_J(MAX)
= 125°C, the maximum R_DS(ON) value can be calculated.
First, calculate the maximum power dissipation:
P_{D(MAX )}
=



T_J(MAX) − T_A(MAX)
R_{(j− a)}



P_{D(MAX )}
=
(125 −
50
60)
=
1.3W
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:
V_OUT • I_{OUT(MAX) , OR 12A}
V_IN(MIN)
We calculate a maximum value for R_DS(ON):
R_DS(ON) (125°C)
<
P_{D(MAX )}
I_IN(MAX) ²
R_DS(ON) (125°C)
1.3W
< _(12A)2
=
0.009Ω
3789f
22