L6926(2004) Ver la hoja de datos (PDF) - STMicroelectronics
Número de pieza
componentes Descripción
Fabricante
L6926 Datasheet PDF : 11 Pages
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L6926
In current mode architectures, when the duty cycle of the application is higher than approximately 50%, a pulse-
by-pulse instability (the so called sub harmonic oscillation) can occur.
To allow loop stability also in these conditions a slope compensation is present. This is realized by reducing the
current flowing through the inductor necessary to trigger the COMP comparator (with a fixed value for the COMP
pin voltage).
With a given duty cycle higher than 50%, the stability problem is particularly present with an higher input voltage
(due to the increased current ripple across the inductor), so the slope compensation effect increases as the input
voltage increases.
From an application point of view, the final effect is that the peak current limit depends both on the duty cycle (if
higher than approximately 40%) and on the input voltage.
4.4 Loop Stability
Since the device is realized with a current mode architecture, the loop stability is usually not a big issue. For
most of the application a 220pF connected between the COMP pin and ground is enough to guarantee the sta-
bility. In case very low ESR capacitors are used for the output filter, such as multilayer ceramic capacitors, the
zero introduced by the capacitor itself can shift at very high frequency and the transient loop response could be
affected. Adding a series resistor to the 220pF capacitor can solve this problem.
The right value for the resistor (in the range of 50K) can be determined by checking the load transient response
of the device. Basically, the output voltage has to be checked at the scope after the load steps required by the
application. In case of stability problems, the output voltage could oscillates before to reach the regulated value
after a load step.
5 ADDITIONAL FEATURES AND PROTECTIONS
5.1 DROPOUT Operation
The Li-Ion battery voltage ranges from approximately 3V and 4.1V-4.2V (depending on the anode material). In
case the regulated output voltage is from 2.5V and 3.3V, it can be that, close to the end of the battery life, the
battery voltage goes down to the regulated one. In this case the device stops to switch, working at 100% of duty
cycle, so minimizing the dropout voltage and the device losses.
5.2 PGOOD (Power Good Output)
A power good output signal is available. The VFB pin is internally connected to a comparator with a threshold
set at 90% of the of reference voltage (0.6V). Since the output voltage is connected to the VFB pin by a resistor
divider, when the output voltage goes lower than the regulated value, the VFB pin voltage goes lower than 90%
of the internal reference value. The internal comparator is triggered and the PGOOD pin is pulled down.
The pin is an open drain output and so, a pull up resistor should be connected to him.
If the feature is not required, the pin can be left floating.
5.3 ADJUSTABLE OUTPUT VOLTAGE
The output voltage can be adjusted by an external resistor divider from a minimum value of 0.6V up to the input
voltage. The output voltage value is given by:
VOUT
=
0.6
⋅
1
+
R-R----21-
5.4 OVP (Overvoltage Protection)
The device has an internal overvoltage protection circuit to protect the load.
If the voltage at the feedback pin goes higher than an internal threshold set 10% (typ) higher than the reference
voltage, the low side power mosfet is turned on until the feedback voltage goes lower than the reference one.
During the overvoltage circuit intervention, the zero crossing comparator is disabled so that the device is also
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