Datasheet
Device description PM6644
14/35 Doc ID 023203 Rev 1
2 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 V
IN
> 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.
V
out
0.9V=
R1
R2
------- - 1+
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