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参数资料
| 型号： | 750108 |
| 厂商： | Wurth Electronics Inc |
| 文件页数： | 9/30页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR LT3748 |
| 设计资源： | LT3748 Design Schematic/Tips |
| 标准包装： | 1 |
| 主要目的： | DC/DC，步降 |
| 输出及类型： | 1，隔离 |
| 功率 - 输出： | 30W |
| 输出电压： | 12V |
| 电流 - 输出： | 2.5A |
| 输入电压： | 22 ~ 75 V |
| 稳压器拓扑结构： | 回扫 |
| 板类型： | 完全填充 |
| 已供物品： | 板 |
| 已用 IC / 零件： | LT3748 |
| 相关产品： | LT3748HMS#TRPBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP LT3748HMS#PBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP LT3748IMS#TRPBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP LT3748IMS#PBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP LT3748EMS#TRPBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP LT3748EMS#PBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP |
| 其它名称： | 732-3308 |
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LT3748
APPLICATIONS INFORMATION
V OUT = V BG ?? FB ?? ?
R REF ? ? ? N PS ? ?
Pseudo-DC Theory of Operation
The R REF and R FB resistors as depicted in the Block Diagram
are external resistors used to program the output voltage.
The LT3748 operates much the same way as traditional
current mode switchers with the exception of the unique
error amplifier which derives its feedback information
from the flyback pulse.
Operation is as follows: when the NMOS output switch
turns off, its drain voltage rises above V IN . The amplitude
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 = D OUT forward voltage
I SEC = Transformer secondary current
ESR = Total impedance of secondary circuit
N PS = Transformer effective primary-to-secondary
turns ratio
The flyback voltage is converted to a current by R FB and
Combining with the previous V FLBK expression yields an
expression for V OUT , in terms of the internal reference,
programming resistors, transformer turns ratio and diode
forward voltage drop:
? R ? ? 1 ?
? ? V F ? I SEC (ESR)
?
Additionally, it includes the effect of nonzero secondary
output impedance (ESR). This term can be assumed to
be zero in boundary control mode.
Temperature Compensation
The first term in the V OUT equation does not have a tem-
perature dependence, but the diode forward drop, V F , has a
significant negative temperature coefficient. To compensate
for this, a positive temperature coefficient current source
is internally connected to the R REF pin. The current is set
by resistor R TC to ground connected between the TC pin
and ground. To cancel the temperature coefficient, the
following equation is used:
R TC =
δ V TC
Q2.NearlyallofthiscurrentflowsthroughresistorR REF to
form a ground-referred voltage. This voltage is fed into the
flyback error amplifier. The flyback error amplifier samples
this output voltage information when the secondary-side
winding current reaches zero. The error amplifier uses a
bandgap voltage, 1.223V, as the reference voltage.
δ V F R FB
δ T R TC
= ?
? R FB
N PS
?
1
? ?
N PS
1
δ V F / δ T
δ V TC
δ T
?
δ T
or,
R FB
≈
N PS
The relatively high gain in the overall loop will then cause
the voltage at the R REF resistor to be nearly equal to the
bandgap reference voltage, V BG . The relationship between
V FLBK and V BG may then be expressed as:
( δ V F / δ T ) = Diode’s forward voltage temperature coef-
ficient
( δ V TC / δ T ) = 1.85mV/°C
? V FLBK ? V BG
? R ? R
? FB ?
? ?=
REF
or
V TC = 0.55V
The resistor value given by this equation should also
be verified experimentally and adjusted, if necessary, to
? R
V FLBK = V BG ?? FB ??
? R
? FB ? ? 1 ? ? V F
= V BG ?
V OUT
? R REF ? ? N PS ?
? ? FB – I SEC (ESR)
R TC ? ? N PS
?
? R REF ?
V BG = Internal bandgap reference
achieve optimal regulation over temperature.
The revised output voltage is as follows:
? ? ?
? ? ?
? V TC ? R
? ??
?
3748fa
9
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