PM6644 datasheet(14/35 Pages) STMICROELECTRONICS

14 / 35 page

Device description

PM6644

14/35

Doc ID 023203 Rev 1

Device description

The PM6644 combines a 350 mA valley current limit step-down regulator with a high

accuracy 3.3 V voltage reference in a small DFN10 3x3 package.

The switching regulator is based on constant-on-time (COT) architecture. This type of

control offers a very fast load transient response with a minimum external component count.

The switching regulator can regulate 3.47 V in Fixed mode (the FB pin tied at VCC) or it can

deliver an adjustable voltage between 0.9 V and 8 V (the FB pin connected to the output

voltage rail through an external resistor divider).

The switching frequency can be adjusted from 200 kHz to 600 kHz by a resistor between

TON and the VIN pin.

The embedded input and output voltage feed-forward provides nearly constant switching

frequency operation.

A pulse skipping technique allows increasing efficiency at very light load.

The switching regulator has protection against overvoltage, undervoltage and overcurrent.

The power MOSFET and switching controller of the switching regulator are supplied by VCC

voltage. An integrated voltage generator from VIN > 6 V provides 3.8 V at the VCC pin when

the BYP pin < 2.4 V if BYP > 3.2 V, the integrated voltage generator is turned off and VCC is

connected to BYP through a MOSFET switch (switch-over function).

An integrated 3.3 V linear regulator (supplied by VCC) provides an accurate 3.3 V output

(REF3).

The PM6644 also provides protection against overtemperature, turning off both switching

regulator and 3.3 V reference.

2.1

Switching regulator

2.1.1

Output voltage set-up

The switching sections can be configured in several ways.

Output voltage is configured with the FB pin. If the FB pin is tied to VCC, the PM6644

regulates 3.47 V. Using an external resistor divider the output can be adjusted following this

equation:

Equation 1

where R1, R2 are the resistors of the FB pin divider. REF is a voltage reference used to

internally generate the 0.9 V threshold used to set the output voltage of the switching

regulator.

2.1.2

Constant-on-time control (COT)

The PM6644 implements a pseudo-fixed frequency algorithm using the COT architecture.

out

0.9V

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