Datasheet

LT3837
19
3837fd
APPLICATIONS INFORMATION
V
IN
R
A
LT3837
(4a) UV Turning ON
UVLO
I
UVLO
R
B
V
IN
R
A
LT3837
(4b) UV Turning OFF (4c) UV Filtering
UVLO
UVLO
R
B
V
IN
R
A2
R
A1
C
UVLO
R
B
3837 F04
I
UVLO
Figure 4
dropping the voltage on UVLO. In this manner, hysteresis
is produced.
Referring to Figure 4, the voltage hysteresis at V
IN
is
equal to the change in bias current times R
A
. The design
procedure is to select the desired V
IN
referred voltage
hysteresis, V
UVHYS
. Then:
R
A
=
V
UVHYS
I
UVLO
where:
I
UVLO
= I
UVLOL
– I
UVLOH
is approximately 3.4µA
R
B
is then selected with the desired turn-on voltage:
R
B
=
R
A
V
IN(ON)
V
UVLO
–1






If we wanted a V
IN
-referred trip point of 8.4V, with 0.3V
of hysteresis (on at 8.4V, off at 8.1V):
R
A
=
0.3V
3.4µA
= 88.2k, use 86.6k
R
B
=
86.6k
8.4V
1.24V
– 1






= 14.99k, use 15k
Even with good board layout, board noise may cause
problems with UVLO. You can filter the divider but keep
large capacitance off the UVLO node because it will slow
the hysteresis produced from the change in bias current.
Figure 4c shows an alternate method of filtering by split-
ting the R
A
resistor with the capacitor. The split should
put more of the resistance on the UVLO side.
Control Loop Compensation
Loop frequency compensation is performed by connect-
ing a capacitor network from the output of the feedback
amplifier (V
C
pin) to ground as shown in Figure 5. Be-
cause of the sampling behavior of the feedback amplifier,
compensation is different from traditional current mode
switcher controllers. Normally only C
VC
is required. R
VC
can be used to add a “zero” but the phase margin improve-
ment traditionally offered by this extra resistor is usually
already accomplished by the nonzero secondary circuit
impedance. C
VC2
can be used to add an additional high
frequency pole and is usually sized at 0.1 times C
VC
.
Figure 5. V
C
Compensation Network
9
R
VC
V
C
C
VC
3825 F05
C
VC2
In further contrast to traditional current mode switchers,
V
C
pin ripple is generally not an issue with the LT3837.
The dynamic nature of the clamped feedback amplifier
forms an effective track/hold type response, whereby the
V
C
voltage changes during the flyback pulse, but is then
held during the subsequent switch-on portion of the next
cycle. This action naturally holds the V
C
voltage stable
during the current comparator sense action (current mode
switching).
AN19 provides a method for empirically tweaking frequency
compensation. Basically, it involves introducing a load
current step and monitoring the response.
Slope Compensation
This part incorporates current slope compensation. Slope
compensation is required to ensure current loop stability
when the DC is greater than 50%. In some switcher con-
trollers, slope compensation reduces the maximum peak
current at higher duty cycles. The LT3837 eliminates this
need by having circuitry that compensates for the slope
compensation so that maximum current sense voltage is
constant across all duty cycles.