Datasheet

'I
LMAX
=
For D > 0.5
'I
LMAX
=
For D < 0.5
L
MIN
f
SWMIN
V
INMAX
(V
INMAX
- V
OUTMIN
)
V
OUTMIN
(V
INMIN
- V
OUTMAX
)
V
OUTMAX
L
MIN
f
SWMIN
V
INMIN
I
IN-RMS
= I
OUT
D(1 - D)
V
DROOP
= 'I
OUTSTEP
x R
ESR
+
L x 'I
OUTSTEP
2
C
OUT
x (V
IN
- V
OUT
)
LM21215
SNVS625E –FEBRUARY 2011–REVISED MARCH 2013 www.ti.com
where
• C
OUT
(F) is the minimum required output capacitance
• L (H) is the value of the inductor
• V
DROOP
(V) is the output voltage drop ignoring loop bandwidth considerations
• ΔI
OUTSTEP
(A) is the load step change
• R
ESR
(Ω) is the output capacitor ESR
• V
IN
(V) is the input voltage
• V
OUT
(V) is the set regulator output voltage (7)
Both the tolerance and voltage coefficient of the capacitor should be examined when designing for a specific
output ripple or transient droop target.
INPUT CAPACITOR SELECTION
Quality input capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the switch
current during the on-time. Additionally, they help minimize input voltage droop in an output current transient
condition. In general, it is recommended to use a ceramic capacitor for the input as it provides both a low
impedance and small footprint. Use of a high grade dielectric for the ceramic capacitor, such as X5R or X7R, will
provide improved over-temperature performance and also minimize the DC voltage derating that occurs with Y5V
capacitors. The input capacitors C
IN1
and C
IN2
should be placed as close as possible to the PVIN and PGND
pins.
Non-ceramic input capacitors should be selected for RMS current rating and minimum ripple voltage. A good
approximation for the required ripple current rating is given by the relationship:
(8)
As indicated by the RMS ripple current equation, highest requirement for RMS current rating occurs at 50% duty
cycle. For this case, the RMS ripple current rating of the input capacitor should be greater than half the output
current. For best performance, low ESR ceramic capacitors should be placed in parallel with higher capacitance
capacitors to provide the best input filtering for the device.
When operating at low input voltages (3.3V or lower), additional capacitance may be necessary to protect from
triggering an under-voltage condition on an output current transient. This will depend on the impedance between
the input voltage supply and the LM21215, as well as the magnitude and slew rate of the output transient.
The AVIN pin requires a 1 µF ceramic capacitor to AGND and a 1Ω resistor to PVIN. This RC network will filter
inherent noise on PVIN from the sensitive analog circuitry connected to AVIN.
PROGRAMMABLE CURRENT LIMIT
A resisitor from the ILIM pin to GND will set the internal current limit on the LM21215. The current limit should be
programmed such that the peak inductor current (I
L
) does not trigger the current limit in normal operation. This
requires setting the resistor from the ILIM pin to GND (R
ILIM
) to the appropriate value to allow the maximum ripple
current, ΔI
LMAX
plus the DC output current through the high-side FET during normal operation. The maximum
ripple current can be described as:
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