LT3837_15 Datasheet, PDF (12/28 Pages) Linear Technology – Isolated No-Opto Synchronous Flyback Controller

LT3837
OPERATION
R1
FB
8
R2 LOAD
COMP I
Q1 Q2
V_FB
Q3
A1
V_FLBK
T1
•
V_{IN •}
•
MP
R_CMPF
50k
SENSE⁺
12
14 R_CMP
13 C_CMP
R_SENSE
3837 F01
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 filtered by
the internal 50k resistor R_CMPF and the external capacitor
on C_CMP. This voltage is then impressed across the exter-
nal R_CMP 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 R_CMP resistor current which affects a corresponding
increase in sensed output voltage, compensating for the
IR drops.
Assuming a relatively fixed power supply efficiency, Eff,
power balance gives:
P_OUT = Eff • P_IN
V_OUT • I_OUT = Eff • V_IN • I_IN
Average primary side current is expressed in terms of
output current as follows:
I_IN = K1•I_OUT
where :
K1=
V_OUT
V_IN •Eff
So the effective change in V_OUT target is:
∆V_OUT
= K1• ∆I_OUT
• ^RSENSE
R_CMP
• R1• N_SF
thus :
∆V_OUT
∆I_OUT
=
K1•
R_SENSE
R_CMP
• R1• N_SF
where:
K1 = dimensionless variable related to V_IN, V_OUT and
efficiency as explained above
R_SENSE = external sense resistor
Nominal output impedance cancellation is obtained by
equating this expression with R_S(OUT):
K1• ^RSENSE
R_CMP
• R1• N_SF
=
ESR +R_DS(ON)
1– DC
Solving for R_CMP gives:
R_CMP
=
K1•
R_SENSE •(1–DC)
ESR +R_DS(ON)
• R1• N_SF
The practical aspects of applying this equation to determine
an appropriate value for the R_CMP resistor are found in the
Applications Information section.
3837fd
12