參數(shù)資料
型號(hào)： LT3837IFE#TRPBF
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, LEAD FREE, PLASTIC, TSSOP-16
文件頁(yè)數(shù)： 11/28頁(yè)
文件大?。?/td> 247K
代理商： LT3837IFE#TRPBF
LT3837
19
3837fc
APPLICATIONS INFORMATION
VIN
RA
LT3837
(4a) UV Turning ON
UVLO
IUVLO
RB
VIN
RA
LT3837
(4b) UV Turning OFF
(4c) UV Filtering
UVLO
RB
VIN
RA2
RA1
CUVLO
RB
3837 F04
IUVLO
Figure 4
dropping the voltage on UVLO. In this manner, hysteresis
is produced.
Referring to Figure 4, the voltage hysteresis at VIN is
equal to the change in bias current times RA. The design
procedure is to select the desired VIN referred voltage
hysteresis, VUVHYS. Then:
R
V
I
A
UVHYS
UVLO
=
where:
IUVLO = IUVLOL – IUVLOH is approximately 3.4μA
RB is then selected with the desired turn-on voltage:
R
V
B
A
IN ON
UVLO
=
()
–1
If we wanted a VIN-referred trip point of 8.4V, with 0.3V
of hysteresis (on at 8.4V, off at 8.1V):
R
V
μA
kuse
k
R
k
V
A ==
=
03
34
88 2
86 6
84
124
.
.,
.
B
V
kuse
k
–
.,
1
14 99
15
=
Even with good board layout, board noise may cause
problems with UVLO. You can lter 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 ltering by split-
ting the RA 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
amplier (VC pin) to ground as shown in Figure 5. Be-
cause of the sampling behavior of the feedback amplier,
compensation is different from traditional current mode
switcher controllers. Normally only CVC is required. RVC
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. CVC2 can be used to add an additional high
frequency pole and is usually sized at 0.1 times CVC.
Figure 5. VC Compensation Network
9
RVC
VC
CVC
3825 F05
CVC2
In further contrast to traditional current mode switchers,
VC pin ripple is generally not an issue with the LT3837.
The dynamic nature of the clamped feedback amplier
forms an effective track/hold type response, whereby the
VC voltage changes during the yback pulse, but is then
held during the subsequent switch-on portion of the next
cycle. This action naturally holds the VC 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.
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