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參數(shù)資料
| 型號： | LLT3837EFE |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 110 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | 4.40 MM, PLASTIC, TSSOP-16 |
| 文件頁數(shù)： | 9/28頁 |
| 文件大小： | 255K |
| 代理商： | LLT3837EFE |
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LT3837
17
3837fa
APPLICATIONS INFORMATION
Size RSENSE using worst-case conditions, minimum LP,
VSENSE and maximum VIN. Continuing the example, let us
assume that our worst-case conditions yield an IPK 10%
above nominal so IPK = 10.41A . If there is a 5% tolerance
on RSENSE and minimum VSENSE = 80mV, then RSENSE
105% = 88mV/10.41A and nominal RSENSE = 8.05mΩ.
Round to the nearest available lower value 8.0m
Ω.
Selecting the Load Compensation Resistor
The expression for RCMP was derived in the Operation
section for primary winding sensing as:
RK
RDC
ESR R
RN
R
CMP
SENSE
DS ON
SP
=
()
+
=
1
–
()
S
S OUT
()
Continuing the example:
K
V
VEff
OUT
IN
1
33
988
0 417
=
= ()=
=
.
%
.
If ESR 0
0 002
0 004
0 417
80
1
..
.
–
()
Ω=
Ω
=
Ω
and R
R
m
DS ON
CMP
0
052
0 002
0 004
22 1
0 33
193
.
..
.
()
Ω+
Ω
=Ω
k
This value for RCMP is a good starting point, but empiri-
cal methods are required for producing the best results.
This is because several of the required input variables are
difcult to estimate precisely. For instance, the ESR term
above includes that of the transformer secondary, but its
effective ESR value depends on high frequency behavior,
not simply DC winding resistance. Similarly, K1 appears
as a simple ratio of VIN to VOUT times (differential) ef-
ciency, but theoretically estimating efciency is not a
simple calculation.
The suggested empirical method is as follows:
1. Build a prototype of the desired supply including the
actual secondary components.
2. Temporarily ground the CCMP pin to disable the load
compensation function. Measure output voltage while
sweeping output current over the expected range.
Approximate the voltage variation as a straight line,
ΔVOUT/ΔIOUT = RS(OUT).
3. Calculate a value for the K1 constant based on VIN, VOUT
and the measured (differential) efciency.
4. Compute:
RK
R
R N
orN
CMP
SENSE
S OUT
SP
SF
= 11
()
5. Verify this result by connecting a resistor of this value
from the RCMP pin to ground.
6. Disconnect the ground short to CCMP and connect the
requisite 0.1
μF lter capacitor to ground. Measure the
output impedance RS(OUT) = ΔVOUT/ΔIOUT with the
new compensation in place. RS(OUT) should have
decreased signicantly. Fine tuning is accomplished
experimentally by slightly altering RCMP. A revised
estimate for RCMP is:
′
=+
RR
R
CMP
S OUT CMP
S OUT
()
1
where R
′CMP is the new value for the load compensation
resistor, RS(OUT)CMP is the output impedance with RCMP
in place and RS(OUT) is the output impedance with no load
compensation (from step 2).
Setting Frequency
The switching frequency of the LT3837 is set by an
external capacitor connected between the OSC pin and
ground. Recommended values are between 200pF and
33pF, yielding switching frequencies between 50kHz and
250kHz. Figure 3 shows the nominal relationship between
external capacitance and switching frequency. Place the
capacitor as close as possible to the IC and minimize OSC
trace length and area to minimize stray capacitance and
potential noise pickup.
You can synchronize the oscillator frequency to an external
frequency. This is done with a signal on the SYNC pin. Set
the LT3837 frequency 10% slower than the desired external
frequency using the OSC pin capacitor, then use a pulse on
the SYNC pin of amplitude greater than 2V and with the
desired period. The rising edge of the SYNC signal initiates
an OSC capacitor discharge forcing primary MOSFET off
(PG voltage goes low). If the oscillator frequency is much
different from the sync frequency, problems may occur
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