PM6680 datasheet(21/49 Pages) STMICROELECTRONICS

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PM6680

Device description

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Device description

The PM6680 is a dual step-down controller dedicated to provide logic voltages for notebook

computers.

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 Figure 27.

The PM6680 regulates two adjustable output voltages: OUT1 and OUT2. The switching

frequency of the two sections can be adjusted to 200/300kHz, 300/400kHz or 400/500kHz

respectively. In order to maximize the efficiency at light load condition, a pulse skipping

mode can be selected.

The PM6680 includes also a 5V linear regulator (LDO5) that can power the switching

drivers. If the output OUT1 regulates 5V, 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. The PM6680 provides protection versus overvoltage,

undervoltage and over temperature as well as power good signals for monitoring purposes.

An external 1.237V reference is available.

7.1

Constant On time PWM control

If the SKIP pin is tied to 5V, the device works in PWM mode. Each power section has an

independent on time control.The PM6680 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. Figure 28 shows the inductor current and the output voltage waveforms

in PWM mode.

OUT

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