Kingbor Technology Co.,Ltd
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KB3426
APPLICATIONS INFORMATION
The selection of COUT is driven by the required effective
series resistance (ESR).
induce ringing at the input, VIN. At best, this ringing can
couple to the output and be mistaken as loop instability. At
worst, a sudden inrush of current through the long wires
can potentially cause a voltage spike at VIN, large enough
to damage the part.
Typically, once the ESR requirement for COUT has been
met, the RMS current rating generally far exceeds the
IRIPPLE(P-P) requirement.Theoutputripple6VOUT isdeter-
mined by:
When choosing the input and output ceramic capacitors,
choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage charac-
teristics of all the ceramics for a given value and size.
£
1
¥
6VOUT 6I ESR +
²
´
¦
L
¤
8fCOUT
where f = operating frequency, COUT = output capacitance
and 6IL = ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since 6IL increases with input voltage.
Output Voltage Programming (kB3426 Only)
In the adjustable version, the output voltage is set by a
resistive divider according to the following formula:
R2
R1
£
¤
¥
´
¦
Aluminum electrolytic and dry tantalum capacitors are
bothavailableinsurfacemountconfigurations.Inthecase
oftantalum,itiscriticalthatthecapacitorsaresurgetested
for use in switching power supplies. An excellent choice is
the AVX TPS series of surface mount tantalum. These are
specially constructed and tested for low ESR so they give
the lowest ESR for a given volume. Other capacitor types
include Sanyo POSCAP, Kemet T510 and T495 series, and
Sprague 593D and 595D series. Consult the manufacturer
for other specific recommendations.
VOUT = 0.6V 1+
²
(2)
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 3.
0.6V ) V
) 5.5V
OUT
R2
V
FB
KB3426
R1
GND
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. Because the
KB3426’s control loop does not depend on the output
capacitor’s ESR for stable operation, ceramic capacitors
can be used freely to achieve very low output ripple and
small circuit size.
Figure 3. Setting the kB3426 Output Voltage
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power times 100%. It is
oftenusefultoanalyzeindividuallossestodeterminewhat
is limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as:
However, care must be taken when ceramic capacitors are
usedattheinputandtheoutput.Whenaceramiccapacitor
is used at the input and the power is supplied by a wall
adapter through long wires, a load step at the output can
Efficiency = 100% – (L1 + L2 + L3 + ...)
whereL1, L2, etc. aretheindividuallossesasapercentage
of input power.
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KINGBOR [ KINGBOR TECHNOLOGY CO ]
