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| 型號： | SC1109 |
| 廠商： | Semtech Corporation |
| 元件分類： | 基準(zhǔn)電壓源/電流源 |
| 英文描述： | Cap-Free, NMOS, 150mA Low Dropout Regulator with Reverse Current Protection |
| 中文描述： | 無電容，NMOS管，150mA的低壓差穩(wěn)壓器的反向電流保護(hù) |
| 文件頁數(shù)： | 15/17頁 |
| 文件大?。?/td> | 223K |
| 代理商： | SC1109 |
SYNCHRONOUS PWM CONTROLLER WITH
DUAL LOW DROPOUT REGULATOR
CONTROLLERS
2000 SEMTECH CORP.
PRELIMINARY - October 16, 2000
SC1109
15
TEL:805-498-2111 FAX:805-498-3804 WEB:http://www.semtech.com
COMPONENT SELECTION
SWITCHING SECTION
OUTPUT CAPACITORS - Selection begins with the most
critical component. Because of fast transient load current
requirements in modern microprocessor core supplies,
the output capacitors must supply all transient load cur-
rent requirements until the current in the output inductor
ramps up to the new level. Output capacitor ESR is
therefore one of the most important criteria. The maxi-
mum ESR can be simply calculated from:
step
current
Transient
=
excursion
voltage
transient
Maximum
Where
=
t
I
≤
t
t
t
ESR
R
V
I
V
Each Capacitor
Total
Technology
C
(μF)
ESR
(m
)
Qty.
Rqd.
C
(μF)
ESR
(m
)
Low ESR Tantalum
330
60
6
2000
10
OS-CON
330
25
3
990
8.3
Low ESR Aluminum
1500
44
5
7500
8.8
(
)
O
IN
t
ESR
I
V
V
C
R
L
≤
OSC
f
IN
L
I
L
4
V
RIPPLE
=
IN
O
)
on
(
DS
2
O
I
COND
P
where
V
V
R
≈
=
cycle
duty
d
d
2
IN
O
SW
10
V
I
P
=
4
f
)
t
t
V
I
P
OSC
f
r
IN
O
SW
+
=
OSC
f
IN
RR
RR
V
Q
P
=
For example, to meet a 100mV transient limit with a
10A load step, the output capacitor ESR must be less
than 10m
. To meet this kind of ESR level, there are
three available capacitor technologies.
The choice of which to use is simply a cost/perfor-
mance issue, with Low ESR Aluminum being the
cheapest, but taking up the most space.
INDUCTOR - Having decided on a suitable type and
value of output capacitor, the maximum allowable value
of inductor can be calculated. Too large an inductor will
produce a slow current ramp rate and will cause the
output capacitor to supply more of the transient load
current for longer - leading to an output voltage sag
below
the
ESR
excursion
The maximum inductor value may be calculated from:
calculated
above.
The calculated maximum inductor value assumes 100%
duty cycle, so some allowance must be made. Choosing
an inductor value of 50 to 75% of the calculated maxi-
mum will guarantee that the inductor current will ramp
fast enough to reduce the voltage dropped across the
ESR at a faster rate than the capacitor sags, hence en-
suring a good recovery from transient with no additional
excursions.
We must also be concerned with ripple current in the
output inductor and a general rule of thumb has been to
allow 10% of maximum output current as ripple current.
Note that most of the output voltage ripple is produced
by the inductor ripple current flowing in the output capac-
itor ESR. Ripple current can be calculated from:
Ripple current allowance will define the minimum permit-
ted inductor value.
POWER FETS - The FETs are chosen based on several
criteria with probably the most important being power
dissipation and power handling capability.
TOP FET - The power dissipation in the top FET is a
combination of conduction losses, switching losses and
bottom FET body diode recovery losses.
a) Conduction losses are simply calculated as:
b) Switching losses can be estimated by assuming a
switching time, if we assume 100ns then:
or more generally,
c) Body diode recovery losses are more difficult to esti-
mate, but to a first approximation, it is reasonable to as-
sume that the stored charge on the bottom FET body
diode will be moved through the top FET as it starts to
turn on. The resulting power dissipation in the top FET
will be:
To a first order approximation, it is convenient to only
consider conduction losses to determine FET suitability.
For a 5V in; 2.8V out at 14.2A requirement, typical FET
losses would be:
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