PM6681A STMicroelectronics, PM6681A Datasheet - Page 23

• Manufacturer Part Number: PM6681A
• Description: Dual synchronous step down controller with adjustable LDO
• Manufacturer: STMicroelectronics

PM6681A
7.6
The low side switch is turned on until the output voltage crosses about Vreg+1 %. Then the
high side MOSFET is turned on for a fixed on time period. Afterwards the low side switch is
enabled until the inductor current reaches the zero-crossing threshold. This keeps the
switching frequency higher than 33 kHz. As a consequence of the control, the regulated
voltage can be slightly higher than Vreg (up to 1 %).
If, due to the load, the frequency is higher than 33 kHz, the device works like in skip mode.
No-audible skip mode reduces audio frequency noise that may occur in pulse skip mode at
very light loads, keeping the efficiency higher than in PWM mode.
Current limit
The current-limit circuit employs a “valley” current-sensing algorithm. During the conduction
time of the low side MOSFET the current flowing through it is sensed. The current-sensing
element is the low side MOSFET on-resistance (
Figure 35. R
An internal 100 µA current source is connected to CSENSE pin and determines a voltage
drop on R
drop, the controller doesn't initiate a new cycle. A new cycle starts only when the sensed
current goes below the current limit.
Since the current limit circuit is a valley current limit, the actual peak current limit is greater
than the current limit threshold by an amount equal to the inductor ripple current. Moreover
the maximum output current is equal to the valley current limit plus half of the inductor ripple
current:
Equation 6
The output current limit depends on the current ripple, as shown in
CSENSE
DS(on)
. If the voltage across the sensing element is greater than this voltage
sensing technique
I
LOAD
(max)
=
I
Lvalley
Figure 35
+
∆
2
I
L
).
Figure 36
Device description
:
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