LT8705_15 Datasheet, PDF (24/44 Pages) Linear Technology – 80V VIN and VOUT Synchronous 4-Switch Buck- Boost DC/DC Controller

LT8705
Applications Information
In order to provide adequate load current at low V_IN volt-
ages in the boost region, L should be at least:
L_(MIN1,BOOST) ≅
V_IN(MIN)
•
⎛
⎜
⎝
DC_{(MAX,M3,BOOST)}
100%
⎞
⎟
⎠
2•
⎛
f •_{⎝⎜⎜}
V_{RSENSE(MAX,BOOST,MAX)}
R_SENSE
–
I_OUT(MAX) • V_OUT(MAX)
V_IN(MIN)
⎞
⎠⎟⎟
where:
DC_{(MAX,M3,BOOST)} is the maximum duty cycle percentage
of the M3 switch (see R_SENSE Selection and Maximum
Current section).
f is the switching frequency
V_{RSENSE(MAX,BOOST,MAX)} is the maximum current sense
voltage in the boost region at maximum duty cycle (see
R_SENSE Selection and Maximum Current section)
Negative values of L_(MIN1,BOOST) indicate that the output
load current I_OUT can’t be delivered in the boost region
because the inductor current limit is too low. If L_(MIN1,BOOST)
is too large or is negative, consider reducing the R_SENSE
resistor value to increase the inductor current limit.
Inductor Selection: Subharmonic Oscillations
The LT8705’s internal slope compensation circuits will
prevent subharmonic oscillations that can otherwise oc-
cur when V_IN/V_OUT is less than 0.5 or greater than 2. The
slope compensation circuits will prevent these oscillations
provided that the inductance exceeds a minimum value
(see the earlier section Inductor Current Sensing and Slope
Compensation for more information). Choose an induc-
tance greater than all of the relevant L_(MIN) limits discussed
below. Negative results can be interpreted as zero.
In the boost region, if V_OUT can be greater than twice V_IN,
calculate L_(MIN2,BOOST) as follows:
L_(MIN2,BOOST) =
⎡
⎣⎢⎢^VOUT(MAX)
–
⎛
⎝⎜⎜
V_IN(MIN) • V_OUT(MAX)
V_OUT(MAX) – V_IN(MIN)
⎞⎤
⎠⎟⎟⎦⎥⎥
•
R_SENSE
H
0.08 • f
In the buck region, if V_IN can be greater than twice V_OUT,
calculate L_(MIN1,BUCK) as follows:
L_(MIN1,BUCK) =
V_IN(MAX)
•
⎛
⎝⎜⎜^1–
V_OUT(MAX)
V_IN(MAX) – V_OUT(MIN)
⎞
⎠⎟⎟
•
R_SENSE
H
0.08 • f
Inductor Selection: Maximum Current Rating
The inductor must have a rating greater than its peak
operating current to prevent inductor saturation resulting
in efficiency loss. The peak inductor current in the boost
region is:
I_L(MAX,BOOST)
≅I_OUT(MAX)
•
V_OUT(MAX)
V_IN(MIN)
⎛
⎜
+^⎜⎜
V_IN(MIN)
•
⎛
⎜
⎝
DC_{(MAX,M3,BOOST}
100%
2•L • f
⎞
⎟
⎠
⎞
⎟
⎟
⎟
A
⎝⎜⎜
⎠⎟⎟
where DC_{(MAX,M3,BOOST)} is the maximum duty cycle
percentage of the M3 switch (see R_SENSE Selection and
Maximum Current section).
8705fe
24
For more information www.linear.com/LT8705