Datasheet

LTC4020
23
4020fd
For more information www.linear.com/LTC4020
I
LIMIT
Pin
Maximum average inductor current can be dynami-
cally adjusted using the I
LIMIT
pin as described in the Pin
Description section. Active servos can also be used to
impose voltages on the I
LIMIT
pin, provided they can only
sink current. Active circuits that source current cannot be
used to drive the I
LIMIT
pin.
using whichever relation yields the largest inductor value
for L
MIN
:
If V
IN
> V
OUT
(step-down conversion):
L
MIN
=
V
OUT
• 1– V
OUT
/ V
IN(MAX)




(
)
f
O
• ∆I
MAX
•I
MAX
If V
IN
< V
OUT
(step-up conversion):
L
MIN
=
V
IN
• 1– V
IN
/ V
OUT(MAX)




( )
f
O
• ∆I
MAX
•I
MAX
For step-down conversion, use the maximum expected
operating voltage for V
IN(MAX)
. If the expected V
OUT
operating range (typically from V
FBMIN
= 2.125V to V
FBMAX
= 2.75V) includes V
IN(MAX)
/2, use that value for V
OUT
. If the
entire operating range is below V
IN(MAX)
/2, use the value
corresponding to V
FBMAX
= 2.75V. If the entire operating
range is above V
IN(MAX)
/2, use the value corresponding
to V
FBMIN
= 2.125V.
For step-up conversion, use the maximum output volt-
age (typically corresponding to pin V
FBMAX
= 2.75V) for
V
OUT(MAX
). If the expected V
IN
operating range includes
V
OUT(MAX)
/2, use that value for V
IN
. If the entire input
operating range is below V
OUT(MAX)
/2, use the maximum
operating voltage for V
IN
. If the entire input operating range
is above V
OUT(MAX)
/2, use the minimum input operating
voltage for V
IN
.
Magnetics vendors typically specify inductors with
maximum RMS and saturation current ratings. Select an
inductor that has a saturation current rating at or above
1.25 • I
MAX
, and an RMS rating above I
MAX
.
Output Decoupling
During periods when the LTC4020 DC/DC converter output
is not connected to the battery through the PowerPath
FET, the system load is driven directly by the converter.
The converter creates large switched currents, so a high
quality, low ESR decoupling capacitor is recommended
to minimize voltage glitches on the V
OUT
supply. Placing
a smaller ceramic decoupling capacitor (0.1µF to 10µF)
in parallel with the output decoupling capacitor is also
recommended for high frequency noise reduction. The
V
OUT
decoupling capacitor (C
VOUT
) absorbs the majority of
applicaTions inForMaTion
Figure 6. Using the I
LIMIT
Pin for Digital Control
of Maximum Average Inductor Current
Figure 7. Driving the I
LIMIT
Pin with a Current Sink
Active Servo Amplifier
4020 F06
10k
LOGIC HIGH = HALF CURRENT
I
LIMIT
LTC4020
4020 F06
SERVO
REFERENCE
I
LIMIT
LTC4020
–
+
Inductor Selection
The primary criterion for inductor value selection in an
LTC4020 charger is the ripple current created in that induc
-
tor. Once the inductance value is determined, an inductor
must also have a saturation current equal to or exceeding
the maximum peak current in the inductor.
An inductor value (L) is calculated given the maximum
desired amount of ripple current (ΔI
MAX
). Maximum
inductor ripple current should generally be in the range
of 0.2 to 0.5 (as a fraction of maximum average inductor
current, I
MAX
). When stepping down, ripple current gets
larger with increased V
IN
, and is maximized when V
OUT
=
V
IN
/2. When stepping up, ripple current gets larger with
increased V
OUT
, and is maximized when V
IN
= V
OUT
/2. The
inductor value must be chosen using the greatest expected
operational difference between these values.
A minimum inductor value for a given maximum ripple
current and operating frequency (f
O
) can be determined
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