ST5R25M データシートの表示（PDF） - STMicroelectronics
部品番号
コンポーネント説明
一致するリスト
ST5R25M Datasheet PDF : 13 Pages
| |||
ST5R00
APPLICATION INFORMATION
PC LAYOUT AND GROUNDING HINTS
The ST5R00 high frequency operation makes PC
layout important for minimizing ground bounce
and noise. Place external components as close
as possible to the device pins. Take care to the
Supply Voltage Source connections that have to
be very close to the Input of the application. Set
the Output Load as close as possible to the
output capacitor. If possible, use a Star ground
connection with the centre point on the Device
Ground pin. To maximize output power and
efficiency and minimize output ripple voltage, use
a ground plane and solder the ICs ground pin
directly to the ground plane.
Remember that the LX Switching Current flows
through the Ground pin, so, in order to minimize
the series resistance that may cause power
dissipation and decrease of the Efficiency
conversion, the Ground pattern has to be as large
as possible.
INDUCTOR SELECTION
An inductor value of 47µH performs well in most
ST5R00 applications. However, the inductance
value is not critical, and the ST5R00 will work
with inductors in the 33µH to 120µH. Smaller
inductance values typically offer a smaller
physical size for a given series resistance,
allowing the smallest overall circuit dimensions.
However, due to higher peak inductor currents,
the output voltage ripple (Ipeak x output filter
capacitors ESR) also tends to be higher. Circuits
using larger inductance values exhibit higher
output current capability and larger physical
dimensions for a given series resistance.
In order to obtain the best application
performances the inductor must respect the
following condition:
- The DC resistance has to be as little as
possible, a good value is <0.25Ω. This choise will
reduce the lost power as heat in the windings.
- The inductor core must not saturate at the
forecast maximum LX current. This is mainly a
function of the Input Voltage, Inductor value and
Output Current. However, it is generally
accettable to bias the inductor into saturation by
as much as 20%, although this will slightly reduce
efficiency. In order to calculate this parameter we
have to distinguish two cases:
1)When a light load is applied on the output
(discontinuous mode operation) the inductor core
must not saturate at
ILX(max)= (VIN x TON)/L.
2)For heavy load (continuos mode operation) the
10/13
inductor core must not saturate at
ILX(max)= (IOUT x T)/TOFF(min) + (VIN x TON)/2L
Where: Vin is the Input Voltage, Ton is the switch
on period (typ. 5µs), L is the inductance value,
Iout is the maximum forecast
Output Current, T=TON+TOFF(min) and TOFF(min) is
the minimum switch off period (typ. 1.7µs),
- Choose an inductance value in the 47µH to
82µH range.
- For application sensitive to Electromagnetic
Interference (EMI), a pot core inductor is
recommended.
DIODE SELECTION
A Schottky diode with an high switching speed
and a very low Forward Voltage (VF) is needed.
Higher VF may cause lost power as heat in the
diode, with a decrease of the Efficiency.
Moreover, since the Output Voltage pin is also
used as the device Supply Voltage, the Start-up
Voltage (see related plots) is strictly due to the
diode Forward Voltage at the rated Forward
Current. A good diode choise is a STPS1L30A
(STM).
INPUT/OUTPUT CAPACITORS SELECTION
The Output Ripple Voltage, as well as the
Efficiency, is strictly related to the behaviour of
these elements. The output ripple voltage is the
product of the peak inductor current and the
output capacitor Equivalent Series Resistance
(ESR). Best performances are obtained with
good high frequency characteristics capacitors
and low ESR. The best compromise for the value
of the Output Capacitance is 47µF Tantalum
Capacitor, Lower values may cause higher
Output Ripple Voltage and lower Efficiency
without compromising the functionality of the
device.
An Input Capacitor is required to compensate, if
present, the series impedance between the
Supply Voltage Source and the Input Voltage of
the Application.
A value of 4.7µF is enough to guarantee stability
for distances less than 2”. It could be necessary
(depending on VIN, VOUT, IOUT values) to
proportionally increase the input capacitor value
up to 100µA for major distances.
In any case we suggest to connect both
capacitors, CIN and COUT, as close as possible to
the device pins.
Share Link: 