LT3837EFE#TRPBF Linear Technology, LT3837EFE#TRPBF Datasheet - Page 12

• Manufacturer Part Number: LT3837EFE#TRPBF
• Description: IC CNTRLR SYNC ISO 16TSSOP
• Manufacturer: Linear Technology
• Type: Flybackr

Specifications of LT3837EFE#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Frequency - Switching: 50kHz ~ 250kHz
• Voltage - Input: 4.5 ~ 20 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Voltage - Output: -
• Power - Output: -

LT3837
OPERATION
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 fi ltered by
the internal 50k resistor R
on C
nal R
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 R
increase in sensed output voltage, compensating for the
IR drops.
Assuming a relatively fi xed power supply effi ciency, Eff,
power balance gives:
12
R1
R2
P
V
OUT
OUT
CMP
COMP I
CMP
CMP
FB
8
LOAD
. This voltage is then impressed across the exter-
= Eff • P
• I
resistor by op amp A1 and transistor Q3. This
resistor current which affects a corresponding
Q3
OUT
14
Figure 1. Load Compensation Diagram
R
CMP
= Eff • V
Q1 Q2
IN
A1
IN
V
FB
+
–
• I
CMPF
IN
13
and the external capacitor
C
R
CMP
50k
CMPF
SENSE
12
V
FLBK
V
IN
+
MP
R
3837 F01
•
•
SENSE
T1
•
Average primary side current is expressed in terms of
output current as follows:
So the effective change in V
where:
Nominal output impedance cancellation is obtained by
equating this expression with R
Solving for R
The practical aspects of applying this equation to determine
an appropriate value for the R
Applications Information section.
K1 = dimensionless variable related to V
effi ciency as explained above
R
I
K
K
R
where
Δ
thus
IN
Δ
Δ
SENSE
1
1
V
CMP
V
I I
=
OUT
=
OUT
•
OUT
R
K I
:
V
R
SENSE
1
IN
=
V
:
CMP
= 1
= 1
= external sense resistor
•
OUT
K
•
K
K
OUT
1
Eff
CMP
•
•
•
R
•
Δ
R
ESR R
R N
SENSE
I
R
gives:
OUT
SENSE
1
CMP
•
+
•
SF
• –
R
(
DS ON
R
•
SENSE
=
1
R N
CMP
(
ESR R
1
OUT
DC
•
CMP
)
)
S(OUT)
1
SF
target is:
•
+
•
–
resistor are found in the
R N
R N
DC
1
DS ON
1
•
•
:
(
SF
SF
)
IN
, V
OUT
3837fc
and