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參數(shù)資料
| 型號(hào)： | ST5R50 |
| 英文描述： | ; No. of Macrocells:64; IC Generic Number:; No. of Outputs:68; Number of Inputs:68; Operating Temp. Max:70 C; Operating Temp. Min:0 C; Propagation Delay:10ns; No. of Pins:44 |
| 中文描述： | 微功耗VFM的STPE型DC / DC變換器 |
| 文件頁(yè)數(shù)： | 10/15頁(yè) |
| 文件大?。?/td> | 553K |
| 代理商： | ST5R50 |
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ST5R00 SERIES
10/15
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 <0.25
. This choice 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 acceptable 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
I
LX(max)
= (V
IN
x T
ON
)/L.
2) For heavy load (continuos mode operation) the inductor core must not saturate at
I
LX(max)
= (I
OUT
x T)/T
OFF(min)
+ (V
IN
x T
ON
)/2L
Where: V
IN
is the Input Voltage, Ton is the switch on period (typ. 5ms), L is the inductance value,
I
OUT
is the maximum forecast Output Current, T = T
ON
+T
OFF(min)
and T
OFF(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 (V
F
) is needed. Higher V
F
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 choice is a STPS1L30A.
INPUT/OUTPUT CAPACITORS SELECTION
The Output Ripple Voltage, as well as the Efficiency, is strictly related to the behavior 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 V
IN
, V
OUT
, I
OUT
values) to proportionally increase the input capacitor value up to 100μA for
major distances.
In any case we suggest to connect both capacitors, C
IN
and C
OUT
, as close as possible to the device pins.
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