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參數(shù)資料
| 型號(hào)： | LT3512MPMS#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 0.8 A SWITCHING REGULATOR, 650 kHz SWITCHING FREQ-MAX, PDSO12 |
| 封裝： | LEAD FREE, PLASTIC, MSOP-16/12 |
| 文件頁(yè)數(shù)： | 25/26頁(yè) |
| 文件大?。?/td> | 244K |
| 代理商： | LT3512MPMS#PBF |
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LT3512
8
3512fa
PSEUDO DC THEORY
In the Block Diagram, RREF (R4) and RFB (R3) are external
resistors used to program the output voltage. The LT3512
operates similar to traditional current mode switchers,
except in the use of a unique error amplifier, which derives
its feedback information from the flyback pulse.
Operation is as follows: when the output switch, Q1, turns
off, its collector voltage rises above the VIN rail. The am-
plitude of this flyback pulse, i.e., the difference between
it and VIN, is given as:
VFLBK = (VOUT + VF + ISEC ESR) NPS
VF = D1 forward voltage
ISEC = Transformer secondary current
ESR = Total impedance of secondary circuit
NPS = Transformer effective primary-to-secondary turns
ratio
RFBandQ2converttheflybackvoltageintoacurrent.Nearly
all of this current flows through RREF to form a ground-
referred voltage. The resulting voltage forms the input
to the flyback error amplifier. The flyback error amplifier
samples the voltage information when the secondary side
winding current is zero. The bandgap voltage, 1.20V, acts
as the reference for the flyback error amplifier.
The relatively high gain in the overall loop will then cause
the voltage at RREF to be nearly equal to the bandgap
reference voltage VBG. The resulting relationship between
VFLBK and VBG approximately equals:
VFLBK
RFB
=
VBG
RREF
or VFLBK = VBG
RFB
RREF
VBG = Internal bandgap reference
Combination of the preceding expression with earlier
derivation of VFLBK results in the following equation:
VOUT = VBG
RFB
RREF
1
NPS
VF ISEC (ESR)
The expression defines VOUT in terms of the internal ref-
erence, programming resistors, transformer turns ratio
and diode forward voltage drop. Additionally, it includes
APPLICATIONS INFORMATION
the effect of nonzero secondary output impedance (ESR).
Boundary control mode minimizes the effect of this im-
pedance term.
Temperature Compensation
The first term in the VOUT equation does not have tem-
perature dependence, but the diode forward drop has a
significant negative temperature coefficient. A positive
temperature coefficient current source connects to the
RREF pin to compensate. A resistor to ground from the
TC pin sets the compensation current.
The following equation explains the cancellation of the
temperature coefficient:
δVF
δT
=
RFB
RTC
1
NPS
δVTC
δT
or,
RTC =
RFB
NPS
1
δVF / δT
δVTC
δT
≈
RFB
NPS
(
δVF/δT) = Diode’s forward voltage temperature coefficient
(
δVTC/δT) = 2mV
VTC = 0.55V
Experimentally verify the resulting value of RTCandadjustas
necessary to achieve optimal regulation over temperature.
The addition of a temperature coefficient current modifies
the expression of output voltage as follows:
VOUT = VBG
RFB
RREF
1
NPS
VF
VTC
RTC
RFB
NPS
–ISEC (ESR)
Output Power
A flyback converter has a complicated relationship be-
tween the input and output current compared to a buck
or a boost. A boost has a relatively constant maximum
input current regardless of input voltage and a buck has a
relatively constant maximum output current regardless of
input voltage. This is due to the continuous nonswitching
behavior of the two currents. A flyback converter has both
discontinuous input and output currents which makes it
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| LT3513EUHF#PBF | 功能描述:IC REG 5-OUT FOR TFT-LCD 38QFN RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - 專用型 系列:- 標(biāo)準(zhǔn)包裝:43 系列:- 應(yīng)用:控制器，Intel VR11 輸入電壓:5 V ~ 12 V 輸出數(shù):1 輸出電壓:0.5 V ~ 1.6 V 工作溫度:-40°C ~ 85°C 安裝類型:表面貼裝 封裝/外殼:48-VFQFN 裸露焊盤 供應(yīng)商設(shè)備封裝:48-QFN（7x7） 包裝:管件 |
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