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參數資料
| 型號： | LT3575IFE#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 4.2 A SWITCHING REGULATOR, 1000 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | 4.40 MM, LEAD FREE, PLASTIC, TSSOP-16 |
| 文件頁數： | 24/24頁 |
| 文件大?。?/td> | 240K |
| 代理商： | LT3575IFE#PBF |
LT3575
9
3575f
APPLICATIONS INFORMATION
ERROR AMPLIFIER—DYNAMIC THEORY
Due to the sampling nature of the feedback loop, there
are several timing signals and other constraints that are
required for proper LT3575 operation.
Minimum Current Limit
The LT3575 obtains output voltage information from the
SW pin when the secondary winding conducts current.
The sampling circuitry needs a minimum amount of time
to sample the output voltage. To guarantee enough time,
a minimum inductance value must be maintained. The
primary side magnetizing inductance must be chosen
above the following value:
LV
t
I
NV
N
H
V
PRI
OUT
MIN
PS
OUT
PS
≥=
.
088
tMIN = minimum off-time, 350ns
IMIN = minimum current limit, 400mA
The minimum current limit is higher than that on the Elec-
trical Characteristics table due to the overshoot caused by
the comparator delay.
Leakage Inductance Blanking
When the output switch rst turns off, the yback pulse
appears. However, it takes a nite time until the transformer
primary side voltage waveform approximately represents
the output voltage. This is partly due to the rise time on
the SW node, but more importantly due to the trans-
former leakage inductance. The latter causes a very fast
voltage spike on the primary side of the transformer that
is not directly related to output voltage (some time is also
required for internal settling of the feedback amplier
circuitry). The leakage inductance spike is largest when
the power switch current is highest.
In order to maintain immunity to these phenomena, a xed
delay is introduced between the switch turn-off command
and the beginning of the sampling. The blanking is internally
set to 150ns. In certain cases, the leakage inductance may
not be settled by the end of the blanking period, but will
not signicantly affect output regulation.
Selecting RFB and RREF Resistor Values
The expression for VOUT, developed in the Operation section,
can be rearranged to yield the following expression for RFB:
R
RN
V
FB
REF
PS
OUT
F
TC
BG
=
+
() +
α
where,
VOUT = Output voltage
VF = Switching diode forward voltage
α = Ratio of Q1, IC to IE, typically 0.986
NPS = Effective primary-to-secondary turns ratio
VTC = 0.55V
The equation assumes the temperature coefcients of
the diode and VTC are equal, which is a good rst-order
approximation.
Strictly speaking, the above equation denes RFB not as
an absolute value, but as a ratio of RREF. So, the next
question is, “What is the proper value for RREF?” The
answer is that RREF should be approximately 6.04k. The
LT3575 is trimmed and specied using this value of RREF.
If the impedance of RREF varies considerably from 6.04k,
additional errors will result. However, a variation in RREF of
several percent is acceptable. This yields a bit of freedom
in selecting standard 1% resistor values to yield nominal
RFB/RREF ratios. The RFB resistor given by this equation
should also be veried experimentally, and adjusted if
necessary for best output accuracy.
Tables 1-4 are useful for selecting the resistor values for
RREF and RFB with no equations. The tables provide RFB,
RREF and RTC values for common output voltages and
common winding ratios.
Table 1. Common Resistor Values for 1:1 Transformers
VOUT (V)
NPS
RFB (kΩ)
RREF (kΩ)
RTC (kΩ)
3.3
1.00
18.7
6.04
19.1
5
1.00
27.4
6.04
28
12
1.00
64.9
6.04
66.5
15
1.00
80.6
6.04
80.6
20
1.00
107
6.04
105
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