LTC4020_15 Datasheet, PDF (23/42 Pages) Linear Technology – 55V Buck-Boost Multi-Chemistry Battery Charger

LTC4020
APPLICATIONS INFORMATION
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):
( ) _LMIN
=
V_OUT
• 1– ^V_OUT / V_IN(MAX)^
f_O • ∆I_MAX •I_MAX
If V_IN < V_OUT (step-up conversion):
LTC4020
I_LIMIT
10k
LOGIC HIGH = HALF CURRENT
4020 F06
Figure 6. Using the I_LIMIT Pin for Digital Control
of Maximum Average Inductor Current
LTC4020
I_LIMIT
SERVO
REFERENCE
4020 F06
Figure 7. Driving the I_LIMIT Pin with a Current Sink
Active Servo Amplifier
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
( ) _{LMIN =} ^VIN ^•
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
4020fb
For more information www.linear.com/LTC4020
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