PM6681A STMicroelectronics, PM6681A Datasheet - Page 21

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

PM6681A
7.4
Figure 32. Circuitry for output ripple compensation
The integrator amplifier generates a current, proportional to the DC errors between the FB
voltage and Vr, which decreases the output voltage in order to compensate the total static
error, including the voltage drop on PCB traces. In addition, CINT provides an AC path for
the output ripple. In steady state, the voltage on COMP1/COMP2 pin is the sum of the
reference voltage Vr and the output ripple (see
COMP pin reaches Vr, a fixed Ton begins and the output increases.
For example, we consider Vout = 5 V with an output ripple of ∆V = 50 mV. Considering C
>> C
C
of the output ripple, with a DC value of Vr + 25 mV = 925 mV.
For more details about the output ripple compensation network, see the paragraph “Closing
the integrator loop” in the design guidelines.
In steady state the FB pin voltage is about Vr and the regulated output voltage depends on
the external divider:
Equation 4
Pulse skip mode
If the SKIP pin is tied to ground, the device works in skip mode.
At light loads a zero-crossing comparator truncates the low-side switch on-time when the
inductor current becomes negative. In this condition the section works in discontinuous
conduction mode. The threshold between continuous and discontinuous conduction mode
is:
INT
FILT
assures an AC path for the output voltage ripple. Then the COMP pin ripple is a replica
, the C
INT
DC voltage drop V
OUT
CINT
=
is about 5 V - Vr + 25 mV = 4.125 V.
Vr
×
Figure 32
⎛
⎜ ⎜
⎝
1
+
R
R
2
1
⎞
⎟ ⎟
⎠
). In fact when the voltage on the
Device description
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INT