LT1507
U
W U U
APPLICATIONS INFORMATION
The term inside the radical has a maximum value of 0.5
when input voltage is twice output and stays near 0.5 for
a relatively wide range of input voltages. It is common
practice, therefore, tosimplyusetheworst-casevalueand
assumethatRMSripplecurrentisonehalfofloadcurrent.
At maximum output current of 1.5A for the LT1507, the
input bypass capacitor should be rated at 0.75A ripple
current. Note however, that there are many secondary
considerations in choosing the final ripple current rating.
These include ambient temperature, average versus peak
load current, equipment operating schedule and required
productlifetime.FormoredetailsseeApplicationNotes19
and 46.
high speed circulating current path shown in Figure 4 and
to make connections to the output capacitor in a manner
that minimizes output ripple and noise. For more details,
see Applications Information section in the LT1376 data
sheet.
SWITCH NODE
L1
5V
HIGH
FREQUENCY
CIRCULATING
PATH
C
3
C
1
V
IN
LOAD
LT1507 • F04
Figure 4. High Speed Switching Path
Input Capacitor Type
Some caution must be used when selecting the type of
capacitor used at the input of regulators. Aluminum
electrolytics are lowest cost, but are physically large to
achieve adequate ripple current rating, and size con-
straints (especially height) may preclude their use.
Ceramic capacitors are now available in larger values and
their high ripple current and voltage rating make them
ideal for input bypassing. Cost is slightly higher and
footprint may also be somewhat larger. Solid tantalum
capacitors are a good choice except that they have a
history of occasional spectacular failures when they are
subjected to very large current surges during power-up.
The capacitors can short and then burn with a brilliant
white light and lots of nasty smoke. This phenomenon
occurs in only a small percentage of units, but it has led
some OEM companies to forbid their use in high surge
applications. The input bypass capacitor of regulators can
see such high surges when a battery or high capacitance
source is connected.
INPUT BYPASSING AND VOLTAGE RANGE
Input Bypass Capacitor
Stepdown converters draw current from the input supply
in pulses. The average height of these pulses is equal to
load current and the duty cycle is equal to VOUT/VIN. Rise
and fall time of the current is very fast. A local bypass
capacitor across the input supply is necessary to ensure
proper operation of the regulator and minimize the ripple
current fed back into the input supply. The capacitor also
forces switching current to flow in a tight local loop,
minimizing EMI.
Do not cheat on the ripple current rating of the input
bypass capacitor, but also don’t get hung up on the value
in microfarads. The input capacitor is intended to absorb
all the switching current ripple, which can have an RMS
value as high as one half of load current. Ripple current
ratings on the capacitor must be observed to ensure
reliable operation. The actual value of the capacitor in
microfarads is not particularly important because at
500kHz, any value above 5µF is essential resistive. Ripple
current rating is the critical parameter. RMS ripple current
can be calculated from:
Several manufacturers have developed a line of solid
tantalum capacitors specially tested for surge capability
(AVX TPS series for instance, see Table 2). Even these
units may fail if the input current surge exceeds a value
equal to the voltage rating of the capacitor divided by 1Ω
(10A for a 10V capacitor). For this reason, AVX recom-
mends using the highest voltage rating possible for the
input capacitor. For equal case size, this means that lower
values of capacitance must be used. As stated above, this
V
(V – V
)
OUT IN
OUT
I
(RMS) = I
OUT
RIPPLE
2
V
IN
14
Linear [ Linear ]
