SS8051T11TB SSC [Silicon Standard Corp.], SS8051T11TB Datasheet - Page 7

• Manufacturer Part Number: SS8051T11TB
• Description: Micro-Power Step-up DC/DC Converter
• Manufacturer: SSC [Silicon Standard Corp.]

Rev.2.01 6/06/2003
Here, V
and t
increase the available output current, but limit it to about
twice the calculated value. When too large an inductor is
used, the output voltage ripple will increase without
providing much additional output current. In conditions of
varying V
minimum V
value can be used to give smaller physical size, but
overshoot of the inductor current will occur (see Current
Limit Overshoot section).
Inductor Selection – SEPIC Regulator
For a SEPIC regulator using the SS8051, the
approximate inductance value can be calculated using
the formula below. As for the boost inductor selection, a
larger or smaller value can be used.
Current Limit Overshoot
The SS8051 uses a constant off-time control scheme;
the MOSFET is turned off after the 350mA current limit is
reached. When the current limit is reached and the
MOSFET actually turns off, there is a 100ns delay time.
During this time, the inductor current exceeds the current
limit by a small amount. The formula below can calculate
the peak inductor current.
Here, V
systems with high input voltages and smaller
inductance values, the current overshoot will be most
apparent. This overshoot can be useful as it helps
increase the amount of available output current. By
using a small inductance value, the current limit
overshoot can be quite high. Even though it is
internally current limited to 350mA, the internal
MOSFET of the SS8051 can handle larger currents
I
OFF
PEAK
D
SAT
= 500ns. A larger value can be used to slightly
IN
= 0.4V (Schottky diode voltage), I
L = 2
= I
, such as battery power applications, use the
IN
= 0.25V (switch saturation voltage). For
LIM
value in the above equation. A smaller
+
V
OUT
V
I
LIM
IN(MAX)
+ V
D
L
– V
x t
SAT
OFF
x 100ns
www.SiliconStandard.com
LIM
= 350mA
without any problem, but the total efficiency will suffer.
For best performance, the I
500mA.
For most SS8051 applications, the high switching
frequency requires high-speed Schottky diodes, such
as the Motorola MBR0530 (0.5A, 30V) with their low
forward voltage drop and fast switching speed. Many
different manufacturers make equivalent parts, but
make sure that the component is rated for at least
0.35A. To achieve high efficiency, the average
current rating of the Schottky diodes should be
greater than the peak switching current. Choose a
reverse breakdown voltage greater than the output
voltage.
The SS8051 supplies energy to the load in bursts by
ramping up the inductor current, then delivering that
current to the load. Using low ESR capacitors will help
Capacitor Selection
Low ESR (Equivalent Series Resistance) capacitors
should be used at the output to minimize the output
ripple voltage and the peak-to-peak transient voltage.
Multilayer ceramic capacitors (MLCC) are the best
choice, as they have a very low ESR and are
available in very small packages. Their small size
makes them a good match with the SS8051’s
SOT-23 package. If solid tantalum capacitors (like the
AVX TPS, Sprague 593D families) or OS-CON
capacitors are used, they will occupy more volume
than ceramic ones and the higher ESR increases the
output ripple voltage. It is important to use a capacitor
with a sufficient voltage rating.
A low ESR surface-mount ceramic capacitor also
makes a good selection for the input bypass capacitor,
which should be placed as close as possible to the
SS8051. A 4.7µF input capacitor is sufficient for
most applications.
Diode Selection
Lowering Output Ripple Voltage
PEAK
is best kept below
SS8051
3 of 4