A6902D View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
A6902D
STMicroelectronics
'A6902D' PDF : 33 Pages View PDF
A6902D
Inductor selection
8.3
Inductor selection
The inductance value fixes the current ripple flowing through the output capacitor.
The minimum inductance value, in order to have the expected current ripple, must be selected.
The target current ripple value is typically in the range of 20% - 40% of the output current.
In the continuous conduction mode (CCM), the required inductance value can be calculated as follow.
(16)
LIND
=
VOUT ∙
1
−
VOUT
VIN
∆ IL ∙ FSW
In order to guarantee a maximum current ripple in every condition, Eq. (16) must be evaluated in case of
maximum input voltage, assuming VOUT fixed.
Increasing the value of the inductance help to reduce the current ripple but, at the same time, strongly impacts the
converter response time to a dynamic load change. The response time is the time required by the inductor to
change its current from the initial to the final value. Until the inductor has finished its charging (or discharging)
time, the output current is supplied (or recovered) by the output capacitors.
Further, if the compensation network is properly designed, during a load variation the device is able to properly
change the duty cycle so improving the control loop transient response.
When this condition is reached the response time is only limited by the time required to change the inductor
current, basically by VIN, VOUT and L.
Minimizing the response time, at the end, can help to decrease the output filter total cost and to reduce the
application area.
8.4
Thermal considerations
RthJ-A is the equivalent static thermal resistance junction to ambient of the device; it can be calculated as the
parallel of many paths of heat conduction from the junction to the ambient.
For this device the path through the pin leads is the one conducting the largest amount of heat. The static Rth J-A
measured on the application is about 110 °C/W.
The junction temperature of device will be estimated as follow:
(17)
TJ = TA + RtℎJA ∙ PTOT
The dissipated power of the device is tied to three different sources:
(18)
PON = RDSON ∙ IOUT 2 ∙ D
Where D is the duty cycle of the application. Note that the duty cycle is theoretically given by the ratio between
VOUT and VIN, but in practice it is substantially higher than this value to compensate for the losses in the overall
application. For this reason, the switching losses related to the RDSON increases compared to an ideal case.
RDSON has a typical value of 0.25 Ω @ 25 °C and increases up to a maximum value of 0.5 Ω @ 125 °C. We can
consider a value of 0.4 Ω.
• Switching losses due to turning ON and OFF. These are derived using the following equation:
(19)
PSW
=
VIN
∙
IOUT
∙
TRISE
+
2
TFALL
∙
FSW
=
VIN
∙
IOUT
∙
TTRAN
∙
FSW
Where TRISE and TFALL represent the switching times of the power element that cause the switching losses when
driving an inductive load (see figure below). TTRAN is the equivalent switching time, approximately 70 ns.
DS5503 - Rev 6
page 17/33
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