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R
C
v 2p f
0
V
OUT
C
O
Gm
R
S
(14)
w
z2
+
1
ǒ
C
O
ESR
Ǔ
+ w
p3
+
1
ǒ
C
C2
R
C
Ǔ
(15)
C
C2
+
C
O
ESR
R
C
(16)
f
z1
+
1
2p C
C
R
C
+
f
0
10
(17)
R
1
+
ǒ
V
OUT
* 1.0
Ǔ
R
2
(18)
TPS51120
SLUS670B – JULY 2005 – REVISED FEBRUARY 2007
2. Choose rectifying (bottom) MOSFET. When R
DS(on)
sensing scheme is selected, the rectifying
MOSFET’s on-resistance is used as this R
S
so that lower R
DS(on)
does not always promise better
performance. In order to clearly detect inductor current, minimum R
S
recommended is to give 15 mV or
larger ripple voltage with the inductor ripple current. This promises smooth transition from CCM to DCM
or vice versa. Upper side of the R
DS(on)
is of course restricted by the efficiency requirement, and usually
this resistance affects efficiency more at high load conditions. When using external resistor current
sensing, there is no restriction for low R
DS(on)
. However, the current sensing resistance R
S
itself affects
the efficiency.
3. Choose output capacitor(s). If organic semiconductor capacitors (OS-CON) or specialty polymer
capacitors (SP-CAP), are used, the ESR to achieve required ripple value at a stable state or transient
load condition determines the amount of capacitor(s) need, and capacitance is then enough to satisfy
stable operation. The peak-to-peak ripple value can be estimated by ESR times the inductor ripple current
for stable state, or ESR times the load current step for a fast transient load response. In case of ceramic
capacitor(s), usually ESR is small enough to meet ripple requirement. On the other hand, transient
undershoot and overshoot driven by output capacitance becomes the key factor to determine the
capacitor(s).
4. Determine f
0
and calculate R
C
using Equation 14 . Note that higher R
C
shows faster transient response
in cost of unstableness. If the transient response is not enough even with high R
C
value, try increasing
the output capacitance. Recommended f
0
is f/4.
5. Calculate C
C2
. The purpose of this capacitance is to cancel the zero caused by ESR of the output
capacitor. If ceramic capacitor are used, there is no need for C
C2
.
6. Calculate C
C
. Purpose of C
C
is to cut DC component to obtain high DC feedback gain. However, as it
causes phase delay, another zero to cancel this effect at f
0
frequency is need. This zero, ω
z1
, is
determined by C
C
and R
C
. Recommended ω
z1
is 10 times lower to the f
0
frequency.
7. In case of adjustable mode, determine the value of R1 and R2. Recommended R2 value is from 10
k Ω to 20 k Ω . Determine R1 using Equation 18 .
20
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