參數(shù)資料
型號(hào)： TPS51511RHLT
廠商： TEXAS INSTRUMENTS INC
元件分類(lèi)： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, 350 kHz SWITCHING FREQ-MAX, PQCC20
封裝： ROHS COMPLIANT, PLASTIC, QFN-20
文件頁(yè)數(shù)： 9/29頁(yè)
文件大?。?/td> 1019K
代理商： TPS51511RHLT
www.ti.com
H(s
H (s)
1
)=
2
3
(6)
H
1(s) +
R
2
R
2 ) R1
(7)
)
Rc
2
Cc
s
1
)(
Ro
Cc
s
1
(
)
Rc
Cc
s
1
(
Ro
Gm
)
s
(
H2
+
+
+
-
=
(8)
H3(s) +
1 ) s
Co
ESR
1 ) s
Co
RL
Rs
(9)
o +
1
2p
R
1
R1 ) R2
Gm
Co
Rc
Rs
+
1
2p
0.75
Vout
Gm
Co
Rc
Rs
(10)
L +
1
I
IND(ripple)
V
IN(max) * VOUT
V
OUT
V
IN(max)
+
3
I
OUT(max)
V
IN(max) * VOUT
V
OUT
V
IN(max)
(11)
(max)
IN
OUT
(max)
IN
)
ON
(
DS
trip
)
peak
(
IND
V
)
V
(
f
L
1
R
V
I
-
+
=
(12)
SLVS735A – FEBRUARY 2007 – REVISED APRIL 2007
APPLICATION INFORMATION (continued)
as a current source with its transconductance of 1/Rs, and the output capacitor represents the modulator
portion. This simplified model is applicable in the frequency space up to approximately half the switching
frequency. Although the inductance has no influence in the small-signal model, it influences the large-signal
model by limiting the slew rate of the current source. This means that the buck converter’s load-transient
response, one of the large signal behaviors, can be improved by using a smaller inductance without affecting
loop stability.
The total open-loop transfer function of the whole system is given by
Assuming RL>>ESR, Ro>>Rc and Cc>>Cc2, the transfer function of each block is
There are three poles and two zeros in H(s). Each pole and zero is given by
ω
p1 = 1/(Cc × Ro)
ω
p2 = 1/(Co × RL)
ω
p1 = 1/(Cc2 × Rc)
ω
z1 = 1/(Cc × Rc)
ω
z2 = 1/(Co × ESR)
Usually, each frequency of the poles and zeros is lower than the 0dB frequency, f0. However, the f0 should be
kept under 1/3 of the switching frequency to avoid the effect of switching-circuit delay. The f0 is given by
Based on the small-signal analysis above, the external components can be selected by the following steps:
1. Choose the inductor.
The inductance value should be determined to give a ripple current of approximately 1/4 to 1/3 of the
maximum output current.
The inductor also needs low DCR to achieve good efficiency, as well as enough room above peak
inductor current before saturation. The peak inductor current can be estimated as follows.
2. Choose the rectifying (bottom) MOSFET.
When the R
DS(ON) sensing scheme is selected, the rectifying MOSFET’s on-resistance is used as this Rs so
that lower R
DS(ON) does not always promise better performance. In order to clearly detect inductor current,
minimum Rs recommended is to give 15 mV or larger ripple voltage with the inductor ripple current. This will
provide smooth transitions from CCM to DCM or vice versa. The 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 Rs itself affects the efficiency.
3. Choose the output capacitor(s).
When using organic semiconductor capacitors (OS-CON) or specialty polymer capacitors (SP-CAP), the ESR
17
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