VIPER22-LED-EV Ver la hoja de datos (PDF) - STMicroelectronics
Número de pieza
componentes Descripción
Fabricante
VIPER22-LED-EV Datasheet PDF : 21 Pages
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VIPer22A-E, VIPer22ADIP-E, VIPer22AS-E
Operations
The Power MOSFET delivers a sense current Is which is proportional to the main current Id.
R2 receives this current and the current coming from the FB pin. The voltage across R2 is
then compared to a fixed reference voltage of about 0.23 V. The MOSFET is switched off
when the following equation is reached:
R2 ⋅ (IS + IFB) = 0.23V
By extracting IS:
IS
=
0----.--2---3----V--
R2
–
IFB
Using the current sense ratio of the MOSFET GID:
ID
=
GID ⋅ IS
=
GID
⋅
⎛
⎝
0----.--2---3----V--
R2
–
IFB⎠⎞
The current limitation is obtained with the FB pin shorted to ground (VFB = 0 V). This leads
to a negative current sourced by this pin, and expressed by:
IFB
=
–0----.--2---3----V--
R1
By reporting this expression in the previous one, it is possible to obtain the drain current
limitation IDlim:
IDlim
=
GI
D
⋅
0.23
V
⋅
⎛
⎝
--1----
R2
+
-R-1---1-⎠⎞
In a real application, the FB pin is driven with an optocoupler as shown on Figure 5. which
acts as a pull up. So, it is not possible to really short this pin to ground and the above drain
current value is not achievable. Nevertheless, the capacitor C is averaging the voltage on
the FB pin, and when the optocoupler is off (start up or short circuit), it can be assumed that
the corresponding voltage is very close to 0 V.
For low drain currents, the formula (1) is valid as long as IFB satisfies IFB < IFBsd, where
IFBsd is an internal threshold of the VIPer22A. If IFB exceeds this threshold the device will
stop switching. This is represented on Figure 12 on page 14, and IFBsd value is specified in the
PWM COMPARATOR SECTION. Actually, as soon as the drain current is about 12 % of
Idlim, that is to say 85 mA, the device will enter a burst mode operation by missing switching
cycles. This is especially important when the converter is lightly loaded.
Doc ID 12050 Rev 2
9/21
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