PM6681A STMicroelectronics, PM6681A Datasheet - Page 18

• Manufacturer Part Number: PM6681A
• Description: Dual synchronous step down controller with adjustable LDO
• Manufacturer: STMicroelectronics

Device description
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Device description
The PM6681A is a dual step-down controller dedicated to provide logic voltages for
industrial automation application and notebook computer.
It is based on a constant on time control architecture. This type of control offers a very fast
load transient response with a minimum external component count. A typical application
circuit is shown in
and OUT2. The switching frequency of the two sections can be adjusted to three different
values. In order to maximize the efficiency at light load condition, a pulse skipping mode can
be selected. The PM6681A includes also a 5 V linear regulator (LDO5) that can power the
switching drivers. If the output OUT1 regulates 5 V, in order to maximize the efficiency in
higher consumption status, the linear regulator can be turned off and their outputs can be
supplied directly from the switching outputs. Moreover, the PM6681A includes also a linear
regulator with an output voltage adjustable from 0.9 V to 3.3 V. It can provide 100 mA of
peak current. The PM6681A provides protection versus overvoltage, undervoltage and
overtemperature as well as
An external 1.237 V reference is available.
Constant on time PWM control
If the SKIP pin is tied to 5 V, the device works in PWM mode. Each power section has an
independent on time control.The PM6681A employees a pseudo-fixed switching frequency,
constant on time (COT) controller as core of the switched mode section. Each power section
has an independent COT control.
The COT controller is based on a relatively simple algorithm and uses the ripple voltage due
to the output capacitor's ESR to trigger the fixed on-time one-shot generator. In this way, the
output capacitor's ESR acts as a current sense resistor providing the appropriate ramp
signal to the PWM comparator. On-time one-shot duration is directly proportional to the
output voltage, sensed at the OUT1/OUT2 pins, and inversely proportional to the input
voltage, sensed at the VIN pin, as follows:
Equation 1
This leads to a nearly constant switching frequency, regardless of input and output voltages.
When the output voltage goes lower than the regulated voltage Vreg, the on-time one shot
generator directly drives the high side MOSFET for a fixed on time allowing the inductor
current to increase; after the on time, an off time phase, in which the low side MOSFET is
turned on, follows.
in PWM mode.
Figure
Figure 30
1. The PM6681A regulates two adjustable output voltages: OUT1
Power Good
shows the inductor current and the output voltage waveforms
T
signals for monitoring purposes.
on
=
K
×
Vout
Vin
PM6681A