PM6644 datasheet(16/35 Pages) STMICROELECTRONICS

16 / 35 page

Device description

PM6644

16/35

Doc ID 023203 Rev 1

Equation 5

The switching frequency is theoretically constant, but in a real application it depends on

parasitic voltage drops that occur during the charging path (high-side switch resistance,

inductor resistance (DCR)) and discharging path (low-side switch resistance, DCR). As a

result, the switching frequency increases as a function of the load current. The following

table shows some examples of switching frequencies that can be selected through the TON

pin (C1 not mounted):

2.1.3

PWM control

Figure 20 shows the simplified schematic of the constant-on-time controller. The COT

architecture uses a minimum OFF-time (TOFFMIN = 500 ns typ.) to allow inductor valley

current sensing on the synchronous switch. A minimum on-time is also introduced to assure

the correct startup sequence.

An adaptive anti-cross conduction algorithm avoids current paths between VIN and GND

during switching transition.

Table 7.

Frequency configurations

TON resistor

R_TON

VOUT = 3.47 V frequency

load = 200 mA (PWM mode)

VOUT = 0. 9 V frequency

load = 200 mA (PWM mode)

2 M

245 kHz

1 M

470 kHz

500 K

260 kHz

250 K

495 kHz

-----------

OUT

0.9V

TON

()

⋅⋅

-----------------------------------------------------------------

OUT

0.9V

TON

()

⋅⋅

-----------------------------------------------------------------

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