Datasheet

LT8619/LT8619-5
17
Rev A
For more information www.analog.com
APPLICATIONS INFORMATION
stay in sleep mode longer between each pulse. This can
be achieved by using a larger value inductor, and should
be considered independent of switching frequency when
choosing an inductor. For example, while a lower inductor
value would typically be used for a high switching fre-
quency application, if high light load efficiency is desired,
a higher inductor value should be chosen.
The optimum inductor for a given application may differ
from the one indicated by this design guide. A larger value
inductor provides a higher maximum load current and
reduces the output voltage ripple. For applications requir-
ing smaller load currents, the value of the inductor may
be lower and the LT8619 may operate with higher ripple
current. This allows you to use a physically smaller induc-
tor, or one with a lower DCR resulting in higher efficiency.
Be aware that low inductance may result in discontinuous
mode operation, which further reduces maximum load
current.
For details of maximum output current and discontinuous
operation, see Analog Devices’s Application Note 44.
Input Capacitor
Step-down regulators draw current from the input sup
-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage rip-
ple at the LT8619 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
In continuous mode, the input capacitor RMS current is
given by
:
I
RMS(MAX)
≈ I
LOAD(MAX)
V
OUT
V
IN
– V
OUT
( )
V
IN
This equation has a maximum RMS current at V
IN
=
2V
OUT
, where I
RMS(MAX)
= I
LOAD(MAX)
/2.
Bypass the input of the LT8619 circuit with a 2.2μF to
10μF ceramic capacitor of X7R or X5R type placed as
close as possible to the V
IN
and GND pin. Y5V types have
poor performance over temperature and applied voltage,
and should not be used. Note that larger input capacitance
is required when a lower switching frequency is used.
If the input power source has high impedance, or there
is significant inductance due to long wires or cables, a
ceramic input capacitor combined with the trace or cable
inductance forms a high quality (underdamped) tank cir-
cuit. If the LT8619 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the LT8619’s voltage rating. This situation is
easily avoided (see Analog Devices Application Note88),
by adding a lossy electrolytic capacitor in parallel with the
ceramic capacitor.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated
by the LT8619 to produce the DC output. In this role it
determines the output ripple, thus low impedance at the
switching frequency is important. The second function
is to store energy in order to satisfy transient loads and
stabilize the LT8619’s control loop. The current slew rate
of a regulator is limited by the inductor and feedback loop.
When the amount of current required by the load changes,
the initial current deficit must be supplied by the output
capacitor until the feedback loop reacts and compensates
for the load changes. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. For good starting values, see the
Typical Application section.
Transient performance can be improved with a higher
value capacitor and the addition of a feedforward capaci-
tor placed between V
OUT
and FB. Increasing the output
capacitance will also decrease the output voltage ripple. A
lower value of output capacitor can be used to save space
and cost but transient performance will suffer and may
cause loop instability. See the Typical Application in this
data sheet for suggested capacitor values.
Ceramic Capacitors
When choosing a capacitor, special attention should be
given to the manufacturer’s data sheet in order to accu
-
rately calculate the effective capacitance under the rel-
evant bias voltage and operating temperature conditions.
Ceramic dielectrics can offer near ideal performance as
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.