Datasheet

LT8705
21
8705fb
For more information www.linear.com/LT8705
applicaTions inForMaTion
The duty cycle of CLKOUT is proportional to the die tem-
perature and can be used to monitor the die for thermal
iss
ues. See the Junction Temperature Measurement section
for more information.
Inductor Current Sensing and Slope Compensation
The LT8705 operates using inductor current mode control.
As described previously in the Power Switch Control sec
-
tion, the LT8705 measures the peak of the inductor current
waveform in the boost region and the valley of the inductor
current waveform in the buck region. The inductor current
is sensed across the R
SENSE
resistor with pins CSP and
CSN. During any given cycle, the peak (boost region) or
valley (buck region) of the inductor current is controlled
by the V
C
pin voltage.
Slope compensation provides stability in constant-
frequency current mode control architectures by prevent-
ing subharmonic oscillations at high duty cycles. This
is ac
c
omplished internally by adding a compensating
ramp to the inductor current signal in the boost region,
or subtracting a ramp from the inductor current signal
in the buck region. At higher duty cycles, this results in
a reduction of maximum inductor current in the boost
region, and an increase of the maximum inductor current
in the buck region. For example, refer to the Maximum
Inductor Current Sense Voltage vs Duty Cycle graph in the
Typical Performance Characteristics section. The graph
shows that, with V
C
at its maximum voltage, the maximum
inductor sense voltage V
RSENSE
is between 78mV and
117mV depending on the duty cycle. It also shows that
the maximum inductor valley current in the buck region
is 86mV increasing to ~130mV at higher duty cycles.
R
SENSE
Selection and Maximum Current
The R
SENSE
resistance must be chosen properly to achieve
the desired amount of output current. Too much resistance
can limit the output current below the application require
-
ments. Start by determining the maximum allowed R
SENSE
resistance in the boost region, R
SENSE(MAX,BOOST)
. Follow
this by finding the maximum allowed R
SENSE
resistance in
the buck region, R
SENSE(MAX,BUCK)
. The selected R
SENSE
resistance must be smaller than both.
Boost Region: In the boost region, the maximum output
current capability is the least when V
IN
is at its minimum
and V
OUT
is at its maximum. Therefore R
SENSE
must be
chosen to meet the output current requirements under
these conditions.
Start by finding the boost region duty cycle when V
IN
is
minimum and V
OUT
is maximum using:
DC
(MAX,M3,BOOST)
≅ 1–
V
IN(MIN)
V
OUT(MAX)




•100%
For example, an application with a V
IN
range of 12V to
48V and V
OUT
set to 36V will have:
DC
(MAX,M3,BOOST)
≅ 1–
12V
36V




•100%=67%
Referring to the Maximum Inductor Current Sense Volt-
age graph in the Typical Performance Characteristics
se
ction, the maximum R
SENSE
voltage at 67% duty cycle
is ≅93mV, or:
V
RSENSE(MAX,BOOST, MAX)
≅93mV
for V
IN
= 12V, V
OUT
= 36V.
Next, the inductor ripple current in the boost region must
be determined. If the main inductor L is not known, the
maximum ripple current ∆I
L(MAX,BOOST)
can be estimated
by choosing ∆I
L(MAX,BOOST)
to be 30% to 50% of the
maximum inductor current in the boost region as follows:
∆I
L (MAX,BOOST)
≅
V
O UT(MAX)
•I
O UT(MAX,BOOST)
V
IN(MIN)
•
100%
%Ripple
–0.5






A
where:
I
OUT(MAX,BOOST)
is the maximum output load current
required in the boost region
%Ripple is 30% to 50%
For example, using V
OUT(MAX)
= 36V, V
IN(MIN)
= 12V,
I
OUT(MAX,BOOST)
= 2A and %Ripple = 40% we can estimate:
∆I
L(MAX,BOOST)
≅
36V • 2A
12V •
100%
40%
–0.5




=3A