L6926D

6/8

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.

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.

ADDITIONAL FEATURES AND PROTECTIONS

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.

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.

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:

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

able to sink current.

THERMAL SHUTDOWN

The device has also a thermal shutdown protection activated when the junction temperature reaches 150 C. In

this case both the high side MOSFET and the low side one are turned off. Once the junction temperature goes

back lower than 95 C, the device restarts the normal operation.

V

OU T

0.6

1

R

2

R

1

-------

+





⋅

=