Page 10 - PEN eBook July 2023

Cover Story — Design

The VDLM 171013801, 171023801 and 171033801 power modules have two modes of operation. The
transition takes place automatically depending on the value of the output current. During light-load
conditions, the module operates in PFM mode. This mode is characterized by a lower switching

frequency to reduce the current consumption. This leads to higher efficiency. In PFM mode, a single
pulse to turn on the high-side switch is generated while monitoring the inductor current. The high-
side switch is kept on until the inductor current hits a preset value of 600 mA (typical).

The high-side switch is turned off and the low-side switch turns on when the inductor current
value is reached. The inductor current decreases, and when it reaches zero, both switches are
turned off (idle time). At this point, the output capacitor solely supplies the load with energy.

While the load is being supplied, the output voltage slowly decreases based on the power consumed.

The module monitors the output voltage ripple, and when a specific limit is met, another pulse is
initiated and the cycle repeats.

When the load current increases, the idle time decreases and the switching frequency increases
until the nominal switching frequency is reached and the module transitions to PWM mode.

The choice of the transition point, where the control mechanism changes from PFM to PWM, is based

on the maximum inductor current of 600 mA (typical). When the module runs in discontinuous-
conduction mode within PFM, the transition point can be calculated approximately using the
following equation:

For the input/output ratio V = 24 V to V = 5 V, f = 700 kHz and an inductance value of L = 4.7 µH,
IN OUT SW
the transition point is about I = 120 mA.
OUT

The efficiency curves in the datasheet of the 171013801, 171023801 and 171033801 show the “typical
performance curves” for different V , V and I combinations.
IN OUT OUT

INFLUENCE OF EFFICIENCY

Battery life calculation

How does the efficiency of a power module affect an application that is supplied by a battery? An
example with a 2,800-mAh battery illustrates the importance of high efficiency for battery life.

10 JULY 2023 | www.powerelectronicsnews.com