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參數(shù)資料
| 型號： | LM2696 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | 3A, Constant On Time Buck Regulator |
| 中文描述： | 第3A，恒定時間降壓穩(wěn)壓器 |
| 文件頁數(shù)： | 9/19頁 |
| 文件大?。?/td> | 833K |
| 代理商： | LM2696 |
Application Information
(Continued)
OUTPUT CAPACITOR
The output capacitor size and ESR have a direct affect on
the stability of the loop. This is because the adaptive on-time
control scheme works by sensing the output voltage ripple
and switching appropriately. The output voltage ripple on a
buck converter can be approximated by assuming that the
AC inductor ripple current flows entirely into the output ca-
pacitor and the ESR of the capacitor creates the voltage
ripple. This is expressed as:
V
OUT
≈
I
L
R
ESR
To ensure stability, two constraints need to be met. These
constraints are the voltage ripple at the feedback pin must be
greater than some minimum value and the voltage ripple
must be in phase with the switch pin.
The ripple voltage necessary at the feedback pin may be
estimated using the following relationship:
V
FB
>
0.057
f
SW
+ 35
Where f
SW
is in kHz and
V
FB
is in mV.
This minimum ripple voltage is necessary in order for the
comparator to initiate switching. The voltage ripple at the
feedback pin must be in-phase with the switch. Because the
ripple due to the capacitor charging and capacitor ESR are
out of phase, the ripple due to capacitor ESR must dominate.
The ripple at the output may be calculated by multiplying the
feedback ripple voltage by the gain seen through the feed-
back resistors. This gain H may be expressed as:
To simplify design and eliminate the need for high ESR
output capacitors, an RC network may be used to feed
forward a signal from the switchpin to the feedback (FB) pin.
See the ‘Ripple Feed Forward’ section for more details.
Typically, the best performance is obtained using POSCAPs,
SP CAPs, tantalum, Niobium Oxide, or similar chemistry
type capacitors. Low ESR ceramic capacitors may be used
in conjunction with the RC feed forward scheme; however,
the feed forward voltage at the feedback pin must be greater
than 30 mV.
RIPPLE FEED FORWARD
An RC network may be used to eliminate the need for high
ESR capacitors. Such a network is connected as shown in
Figure 1
.
The value of R
ff
should be large in order to prevent any
potential offset in V
OUT
. Typically the value of R
ff
is on the
order of 1 M
and the value of R
FB1
should be less than
10 k
. The large difference in resistor values minimizes
output voltage offset errors in DCM. The value of the capaci-
tor may be selected using the following relationship:
Where the on-time (T
ON_MIN
) is in μs, and the resistance
(R
ff
) is in M
.
FEEDBACK RESISTORS
The feedback resistors are used to scale the output voltage
to the internal reference value such that the loop can be
regulated. The feedback resistors should not be made arbi-
trarily large as this creates a high impedance node at the
feedback pin that is more susceptible to noise. Typically,
R
is on the order of 1 k
. To calculate the value of R
FB1
,
one may use the relationship:
Where V
is the internal reference voltage that can be
found in the electrical characteristics table (1.254V typical).
The output voltage value can be set in a precise manner by
taking into account the fact that the reference voltage is
regulating the bottom of the output ripple as opposed to the
average value. This relationship is shown in
Figure 2
.
20153428
FIGURE 1. RC Feed Forward Network
L
www.national.com
9
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