PM6675AS datasheet(16/48 Pages) STMICROELECTRONICS

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Device description

PM6675AS

16/48

Device description

The PM6675AS combines a single high efficiency step-down controller and an independent

Low Drop-Out (LDO) linear regulator in the same package.

The switching controller section is a high-performance, pseudo-fixed frequency, Constant-

On-Time (COT) based regulator specifically designed for handling fast load transient over a

wide range of input voltage.

The switching section output can be easily set to a fixed 1.5 V voltage without additional

components or adjusted in the 0.6 V to 3.3 V range using an external resistor divider. The

Switching Mode Power Supply (SMPS) can handle different modes of operation in order to

minimize noise or power consumption, depending on the application needs. Selectable low-

consumption and low-noise modes allow the highest efficiency and a 33 kHz minimum

switching frequency respectively at light loads.

The current sensing is lossless, based on the Low-Side MOSFET turn-on resistance.

The input of the LDO can be either the switching section output or a lower voltage rail in

order to reduce the total power dissipation. Linear regulator stability is achieved by filtering

its output wit

h a ceramic capacitor (20 µF or greater). The LDO linear regulator can sink and

source up to 2 Apk.

Two fixed current limit (±1A-±2A) can be chosen.

An active soft-end is independently performed on both the switching and the linear

regulators outputs when disabled.

7.1

Switching section - constant on-time pwm controller

The PM6675AS employes a pseudo-fixed frequency, Constant On-Time (COT) controller as

the core of the switching section. As well known, the COT controller concerns of a relatively

simple algorithm and uses the ripple voltage derived across the output capacitor ESR to

trigger the On-Time one-shot generator. In this way, the output capacitor ESR acts as a

current sense resistor providing the appropriate ramp signal to the PWM comparator.

Nearly constant switching frequency is achieved by the system loop in steady-state

operating conditions by varying the On-Time duration, avoiding thus the need for a clock

generator. The On-Time one shot duration is directly proportional to the output voltage,

sensed at VSNS pin, and inversely proportional to the input voltage, sensed at the VOSC

pin, as follows:

Equation 1

where KOSC is a constant value (130 ns typ.) and τ is the internal propagation delay (40ns

typ.). The one-shot generator directly drives the high-side MOSFET at the beginning of

each switching cycle allowing the inductor current to increase; after the On-Time has

expired, an Off-Time phase, in which the low-side MOSFET is turned on, follows.

OSC

SNS

OSC

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