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PM6675 Datasheet(PDF) 19 Page - STMicroelectronics

Part # PM6675
Description  Very fast load transient response using constant on-time control loop
PDF  47 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

PM6675 Datasheet(HTML) 19 Page - STMicroelectronics

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PM6675
Device description
19/47
The design of the external feedback network depends on the output voltage ripple. If the
ripple is higher than approximately 20mV, the correct CINT capacitor is usually enough to
keep the loop stable. The stability of the system depends firstly on the output capacitor zero
frequency.
The following condition must be satisfied:
Equation 6
where k is a fixed design parameter (k > 3). It determinates the minimum integrator
capacitor value:
Equation 7
where gm = 50
µs is the integrator trans conductance.
If the ripple on the COMP pin is greater than the integrator 150mV, the auxiliary capacitor
CFILT can be added. If q is the desired attenuation factor of the output ripple, CFILT is given
by:
Equation 8
In order to reduce the noise on the COMP pin, it is possible to add a resistor RINT that,
together with CINT and CFILT, becomes a low pass filter. The cutoff frequency fCUT must be
much greater (10 or more times) than the switching frequency:
Equation 9
If the ripple is very small (lower than approximately 20mV), a different compensation
network, called "Virtual-ESR" Network, is needed. This additional circuit generates a
triangular ripple that is added to the output voltage ripple at the input of the integrator. The
complete control scheme is shown in Figure 8.
ESR
C
2
k
f
k
f
out
Zout
SW
⋅
⋅
π
=
⋅
>
Vout
Vr
f
k
f
2
g
C
Zout
SW
m
INT
⋅
⎟
⎠
⎞
⎜
⎝
⎛
−
⋅
π
>
q
)
q
1
(
C
C
INT
FILT
−
⋅
=
FILT
INT
FILT
INT
CUT
INT
C
C
C
C
f
2
1
R
+
⋅
⋅
⋅
π
=



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