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| 型号： | LT3512MPMS#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 8/26页 |
| 文件大小： | 0K |
| 描述： | IC REG FLYBK ISO ADJ .42A 16MSOP |
| 标准包装： | 37 |
| 类型： | 回扫，隔离 |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 可调 |
| 输入电压： | 4.5 V ~ 100 V |
| PWM 型： | 电流模式，混合 |
| 频率 - 开关： | 40kHz ~ 650kHz |
| 电流 - 输出： | 420mA |
| 同步整流器： | 无 |
| 工作温度： | -55°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 16-TFSOP（0.118"，3.00mm），12 引线 |
| 包装： | 管件 |
| 供应商设备封装： | 16-MSOP |
LT3512
APPLICATIONS INFORMATION
PSEUDO DC THEORY
In the Block Diagram, R REF (R4) and R FB (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 V IN rail. The am-
plitude of this flyback pulse, i.e., the difference between
it and V IN , is given as:
V FLBK = (V OUT + V F + I SEC ? ESR) ? N PS
V F = D1 forward voltage
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 V OUT 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
R REF pin to compensate. A resistor to ground from the
T C pin sets the compensation current.
The following equation explains the cancellation of the
temperature coefficient:
= ? FB ?
I SEC =Transformersecondarycurrent
ESR = Total impedance of secondary circuit
d V F
d T
R
R TC
1
N PS
?
d V TC
d T
or,
≈ FB
N PS =Transformereffectiveprimary-to-secondaryturns
ratio
R TC =
? R FB
N PS
?
1
d V F / d T
?
d V TC
d T
R
N PS
or V FLBK = V BG ?
? ? = R
V OUT = V BG ?
? R FB ? ? 1 ?
? R REF ? ? ? ? N PS ? ?
? ? TC ? ? FB – I SEC (ESR)
V OUT = V BG ?
? R FB ? ? 1 ?
? R REF ? ? ? ? N PS ? ?
R FB andQ2converttheflybackvoltageintoacurrent.Nearly
all of this current flows through R REF 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 R REF to be nearly equal to the bandgap
reference voltage V BG . The resulting relationship between
V FLBK and V BG approximately equals:
? V FLBK ? V BG ? R FB ?
? ?
? R FB REF ? R REF ?
V BG = Internal bandgap reference
Combination of the preceding expression with earlier
derivation of V FLBK results in the following equation:
? V F ? I SEC (ESR)
The expression defines V OUT in terms of the internal ref-
erence, programming resistors, transformer turns ratio
and diode forward voltage drop. Additionally, it includes
( d V F / d T ) = Diode’s forward voltage temperature coefficient
( d V TC / d T) = 2mV
V TC = 0.55V
Experimentally verify the resulting value of R TC and adjust as
necessary to achieve optimal regulation over temperature.
The addition of a temperature coefficient current modifies
the expression of output voltage as follows:
? V F
? V ? R
? R TC ? N PS
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
3512fb
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