AN937 Datasheet, PDF (15/29 Pages) STMicroelectronics – The L4971 is a 1.5 A monolithic dc-dc converter

AN937
2.7
Power supply, UVLO and voltage reference
Power stage
The power stage is realized by a N-channel D-MOS transistor with a Vdss in excess of 60 V
and typ R_dson of 290 mΩ (measured at the device pins).
Minimizing the R_dson, means also minimize the conduction losses.
But also the switching losses have to be taken into consideration. mainly for the two
following reasons:
a) They are affecting the system efficiency and the device power dissipation
b) Because they generate EMI.
2.8
Turn - on
At turn-on of the power element, or better, the rise time of the current (di/dt) at turn-on is the
most critical parameter to compromise.
At a first approach, it looks that faster is the rise time and lower are the turn-on losses.
It’s not completely true.
There is a limit, and it’s introduce by the recovery time of the recirculation diode.
Above this limit, about 100 A/usec, only disadvantages are obtained:
1. Turn-on overcurrent is decreasing efficiency and system reliability
2. Big EMI increase.
The L4971 has been developed with a special focus on this dynamic area.
An innovative and proprietary gate driver, with two different timings, has been introduced.
When the diode reverse voltage is reaching about 3 V, the gate is sourced with low current
(see Figure 20) to assure the complete recovery of the diode without generating unwanted
extra peak currents and noise. After this threshold, the gate drive current is quickly
increased, producing a fast rise time till the peak current, so maintaining the efficiency very
high.
Figure 19. Turn on and Turn off (pin 2, 3)
Doc ID 5655 Rev 15
15/29