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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)： | 4/28頁 |
| 文件大?。?/td> | 255K |
| 代理商： | LLT3837EFE |
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LT3837
12
3837fa
OPERATION
T1
MP
RCMPF
50k
VIN
VFLBK
R2
LOAD
COMP I
R1
FB
VFB
Q1 Q2
RCMP
CCMP
RSENSE
SENSE+
3837 F01
Q3
–
+
A1
8
14
13
12
Figure 1. Load Compensation Diagram
Figure 1 shows the block diagram of the load compensa-
tion function. Switch current is converted to voltage by the
external sense resistor, averaged and lowpass ltered by
the internal 50k resistor RCMPF and the external capacitor
on CCMP. This voltage is then impressed across the exter-
nal RCMP resistor by op amp A1 and transistor Q3. This
produces a current at the collector of Q3 that is subtracted
from the FB node. This action effectively increases the
voltage required at the top of the R1/R2 feedback divider
to achieve equilibrium.
The average primary side switch current increases to
maintain output voltage regulation as output loading
increases. The increase in average current increases
the RCMP resistor current which affects a corresponding
increase in sensed output voltage, compensating for the
IR drops.
Assuming a relatively xed power supply efciency, Eff,
power balance gives:
POUT = Eff PIN
VOUT IOUT = Eff VIN IIN
Average primary side current is expressed in terms of
output current as follows:
IK I
where
K
V
VEff
IN
OUT
IN
=
1
:
So the effective change in VOUT target is:
Δ=
Δ
VK
I
R
RN
thus
V
OUT
SENSE
CMP
SF
OUT
11
:
II
K
R
RN
OUT
SENSE
CMP
SF
= 11
where:
K1 = dimensionless variable related to VIN, VOUT and
efciency as explained above
RSENSE = external sense resistor
Nominal output impedance cancellation is obtained by
equating this expression with RS(OUT):
K
R
RN
ESR R
DC
SENSE
CMP
SF
DS ON
11
1
–
()
=
+
Solving for RCMP gives:
RK
RDC
ESR R
RN
CMP
SENSE
DS ON
SF
=
()
+
1
–
()
The practical aspects of applying this equation to determine
an appropriate value for the RCMP resistor are found in the
Applications Information.
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