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ISL85012 Datasheet(PDF) 16 Page - Renesas Technology Corp

Part No. ISL85012
Description  12A, 3.8V to 18V Input, Synchronous Buck Regulator
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Maker  RENESAS [Renesas Technology Corp]
Homepage  http://www.renesas.com
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ISL85012 Datasheet(HTML) 16 Page - Renesas Technology Corp

 
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ISL85012
FN8677 Rev.2.00
Page 16 of 19
Mar 17, 2017
The simplified transfer function is derived in Equation 14.
where:
Note that Co is the actual capacitance seen by the regulator,
which may include ceramic high frequency decoupling and bulk
output capacitors. Ceramic may have to be derated by
approximately 40% depending on dielectric, voltage stress, and
temperature.
Usually, a type II compensation network is used to compensate
the peak current mode control converter. Figure 38 shows a
typical type II compensation network and its transfer function is
expressed in Equation 16. The frequency response is shown in
Figure 39.
where:
Design example: VIN = 12V, VO = 1.8V, IO = 10A, fSW = 600kHz,
R1 = 200kΩ, R2 = 100kΩ, Co = 3x100µF/3mΩ 6.3V ceramic
(actually ~150µF), L = 0.68µH.
Select fc = 80kHz. The gain of the Gp(s)xAv(s) should has a unity
gain at crossover frequency. Thus, R3 can be derived as:
Select 800kΩ for R3. Place the zero fz1 around the pole fp to
achieve -20db/dec roll off.
where Rc is the ESR of the output capacitor.
Select 30pF for C2. Zero fz2 is a phase boost zero to increase the
phase margin. Place it between fc and 1/2 switching frequency.
In this case, 4.7pF capacitor is selected and the zero is placed at
fz2:
The calculated values for R1, R2, C1, and R3, C2 match with the
1.8V output application in the recommended design with internal
compensation shown in Table 1 on page 2. Do not select
resistance higher than 370kΩ for R1 in real applications to avoid
parasitic impaction.
In practice, it is recommended to select lower resistance for
R1/R2 and R3 in the external compensation applications.
Usually, 10 times lower compared with the internal
compensation is a good start.
FIGURE 38. TYPE II COMPENSATION NETWORK
FIGURE 37. POWER TRAIN SMALL FREQUENCY RESPONSE
Gdc
fp
fz
fc
Gp S

o
comp
-----------------
Gdc
1
S
z
------
+
1
S
p
-------
+
-----------------
=
=
(EQ. 14)
Gdc
Ro
Rt
-------
z
;
=
2
fz
1
RcxCo
------------------
p
;
2
fp
1
Ro Rc
+
xC
o
-------------------------------------
==
=
=
(EQ. 15)
Vref
Vfb
Vo
R1
R2
C1
R3
C2
Vcomp
Av S

comp
o
-----------------
1
S
cz1
-------------
+

 1
S
cz2
-------------
+


SC2R1
---------------------------------------------------------
=
=
(EQ. 16)
cz1
2
f
z1
1
R3C2
---------------
cz2
2
f
z2
=
1
R1C1
--------------- fpc
1
2
R
1C2
----------------------
==
=
=
FIGURE 39. POWER TRAIN FREQUENCY RESPONSE
fc
fz1
fpc
fz2
R3/R1
R3
2
f
cCoRtR1
829k
==
(EQ. 17)
C2
Ro Rc
+
xC
o
R3
--------------------------------------
29pF
=
=
(EQ. 18)
fz2
1
2
R
1C1
----------------------
169kHz
==
(EQ. 19)


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