Datasheet

LMR70503
SNVS850A –JUNE 2012–REVISED APRIL 2013 www.ti.com
The output capacitor is responsible for filtering the output voltage and suppling load current during transients and
during the power diode off-time. Best performance is achieved with ceramic capacitors. For most applications, a
minimum value of 22 μF, 6.3 V capacitor is required at the output of the LMR70503. The percentage of ripple
coupled to the FB node can be found by
RIPPLE PERCENTAGE = V
REF
/ ( |V
OUT
| + V
REF
)
where
• |V
OUT
| is the magnitude of the output voltage
• V
REF
is the reference voltage (4)
With lower magnitude V
OUT
, a higher percentage of output voltage ripple is coupled to the FB node. Output
voltage ripple is also coupled to the FB node via the feed-forward capacitor C
FF
. Excessive ripple at the FB node
may trigger peak current limit modulation causing unstable operation. Higher output capacitance is needed at
lower magnitude output voltage. For V
OUT
= -0.9 V, a minimum of 44 μF, 6.3 V capacitor is required. Avoid using
too much capacitance at C
FF
.
A capacitor between V
IN
and V
OUT
also can be used to provide high frequency bypass. This capacitor is
equivalent to the output capacitors in the small signal model. It also reduces the output voltage ripple if
sufficiency capacitance is used. The voltage rating for this capacitor should be higher than V
IN
+ |V
OUT
|.
All ceramic capacitors have large voltage coefficients, in addition to normal tolerances and temperature
coefficients. To help mitigate these effects, multiple capacitors can be used in parallel to bring the minimum
capacitance up to the desired value. This may also help with RMS current constraints by sharing the current
among several capacitors. With the LMR70503, ceramic capacitors rated at 6.3 V, or higher, are suitable for all
input and output voltage combinations. Many times it is desirable to use an electrolytic capacitor on the input, in
parallel with the ceramics. The moderate ESR of this capacitor can help to damp any ringing on the input supply
caused by long power leads. This method can also help to reduce voltage spikes that may exceed the maximum
input voltage rating of the LMR70503.
Power Inductor Selection
The power inductor selection is critical to the operation of the LMR70503. It affects the efficiency, the operation
mode transition point, the maximum loading capability and the size / cost of the solution. A 4.7 μH or 6.8 μH
inductor is recommended for most LMR70503 applications. The maximum loading capability is reduced with
smaller inductance value. The no load V
OUT
offset is higher at low V
OUT
with smaller inductance value, due to
higher peak current with the same T
ON-MIN
. Higher inductance value usually comes with higher DCR with the
same size and cost. Higher DCR will reduce the efficiency especially at heavy load.
The inductor must be rated above the maximum peak current limit to prevent saturation. Good design practice
requires that the inductor rating be adequate for the maximum I
PEAK-MAX
over V
IN
and temperature, plus some
safety margin. If the inductor is not rated for the maximum expected current, saturation at high current may
cause damage to the LMR70503 and/or the power diode. The DCR of the inductor should be as small as
possible with given size / cost constrains to achieve optimal efficiency.
Power Diode Selection
A Schottky type power diode is required for all LMR70503 applications. The parameters of interests include the
reverse voltage rating, the DC current rating, the repetitive peak current rating, the forward voltage drop, the
reverse leakage current and the parasitic capacitance. In a buck-boost, this diode sees a reverse voltage of :
V
R-DIODE
= |V
OUT
| + V
IN
(5)
The reverse breakdown voltage rating of the diode should be selected for this value, plus safety margin. A good
rule of thumb is to select a diode with a reverse voltage rating of 1.3 times this maximum. Select a diode with a
DC current rating at least equal to the maximum load current that will be seen in the application and the
repetitive peak current rating higher than I
PEAK-MAX
over V
IN
and temperature. The forward voltage drop of the
power diode is a big part of the power loss in a buck-boost converter. It is preferred to be as low as possible. The
reverse leakage current and the parasitic capacitance are also part of the power losses in the converter, but
usually less pronounced than the forward voltage drop loss. Pay attention to the temperature coefficients of all
the parameters. Some of them may vary greatly over temperature and may adversely affect the efficiency over
temperature.
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