參數(shù)資料
型號(hào)： LLT3837EFE
廠商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, 110 kHz SWITCHING FREQ-MAX, PDSO16
封裝： 4.40 MM, PLASTIC, TSSOP-16
文件頁(yè)數(shù)： 7/28頁(yè)
文件大小： 255K
代理商： LLT3837EFE
LT3837
15
3837fa
APPLICATIONS INFORMATION
It then reverts to a potentially stable state whereby the
top of the leakage spike is the control point, and the
trailing edge of the leakage spike triggers the collapse
detect circuitry. This typically reduces the output voltage
abruptly to a fraction, roughly one-third to two-thirds of
its correct value.
Once load current is reduced sufciently, the system snaps
back to normal operation. When using transformers with
considerable leakage inductance, exercise this worst-case
check for potential bistability:
1. Operate the prototype supply at maximum expected
load current.
2. Temporarily short-circuit the output.
3. Observe that normal operation is restored.
If the output voltage is found to hang up at an abnormally
low value, the system has a problem. This is usually evident
by simultaneously viewing the primary side MOSFET drain
voltage to observe rsthand the leakage spike behavior.
A nal note—the susceptibility of the system to bistable
behavior is somewhat a function of the load current/volt-
age characteristics. A load with resistive—i.e., I = V/R
behavior—is the most apt to be bistable. Capacitive loads
that exhibit I = V2/R behavior are less susceptible.
Secondary Leakage Inductance
Leakage inductance on the secondary forms an inductive
divider on the transformer secondary, reducing the size
of the feedback yback pulse. This increases the output
voltage target by a similar percentage.
Note that unlike leakage spike behavior, this phenomenon
is independent of load. Since the secondary leakage in-
ductance is a constant percentage of mutual inductance
(within manufacturing variations), the solution is to adjust
the feedback resistive divider ratio to compensate.
Winding Resistance Effects
Primary or secondary winding resistance acts to reduce
overall efciency (POUT/PIN).Secondarywindingresistance
increases effective output impedance degrading load regu-
lation. Load compensation can mitigate this to some extent
but a good design keeps parasitic resistances low.
Bilar Winding
A bilar or similar winding is a good way to minimize
troublesome leakage inductances. Bilar windings also
improve coupling coefcients and thus improve cross
regulation in multiple winding transformers. However,
tight coupling usually increases primary-to-secondary
capacitance and limits the primary-to-secondary break-
down voltage, so it isn’t always practical.
Primary Inductance
The transformer primary inductance, LP, is selected based
on the peak-to-peak ripple current ratio (X) in the trans-
former relative to its maximum value. As a general rule,
keep X in the range of 50% to 70% ripple current (i.e., X =
0.5 to 0.7). Higher values of ripple will increase conduction
losses, while lower values will require larger cores.
Ripple current and percentage ripple is largest at minimum
duty cycle; in other words, at the highest input voltage.
LP is calculated from:
L
VDC
fX
P
V
P
IN MAX
MIN
OSC
MAX
IN
IN MAX
= () =
()
2
D
DC
Eff
fX
P
MIN
OSC
MAX
OUT
()2
where:
fOSC is the OSC frequency
DCMIN is the DC at maximum input voltage
XMAX is ripple current ratio at maximum input voltage
Continuing with the 9V to 3.3V example, let us assume a
10A output, 9V to 18V input power with 88% efciency.
Using X = 0.7, and fOSC = 200kHz:
P
A
W
DC
NV
V
IN
MIN
IN MAX
OUT
=
=
+
33 10
88
37 5
1
.
%
.
()
==
+
=
()
1
3
18
33
35 5
18
0 355
200
0
2
.
.%
.
L
V
kHz
P
7
737 5
78
.
W
H
=μ
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