LT3585-0/LT3585-1
LT3585-2/LT3585-3
18
3585f
TYPICAL APPLICATIO S
U
5
46
1 2 3
U1
V
IN
3585 F15
TO
DONE
R3
100k
1/10W
0402
R1
5k
1/10W
0402
R
T
100k
C
T
0.1 F
ENABLE
TO
CHRG/IADJ
U1: PANASONIC UP04979 COMPOSITE TRANSISTORS
Figure 15. Auto Refresh Application
V
OUT
100V/DIV
CHRG/IADJ
2V/DIV
ENABLE
2V/DIV
2sec/DIVLT3585-1
C
OUT
= 50µF
ENABLE < 0.3V
ENABLE > 1.1V
NORMAL OP.
AUTO
REFRESH
AUTO
REFRESH
ENABLE
<0.3V
ENABLE
>1.1V
ENABLE < 0.3V
3585 F16
V
OUT
2V/DIV
AC RIPPLE
CHRG/IADJ
2V/DIV
200ms/DIVLT3585-1
C
OUT
= 50µF
3585 F17
Figure 16. Auto Refresh Basic Operation
Figure 17. V
OUT
AC Ripple in Auto Refresh Mode
The LT3585 series can be auto-refreshed using the addi-
tional circuitry shown in Figure 15 with its basic operation
shown in Figure 16. The ENABLE pin is used to enable
or disable the auto-refresh charging mode. Without an
auto-refresh circuit, the output voltage will droop due to
output capacitor and output diode leakage currents. The
circuit in Figure 15 uses the DONE and CHRG/IADJ pins
to form an open-loop control scheme. The output voltage
target is sensed through the DONE pin with the PFET of
U1, Panasonic UP04979 composite transistor. When the
DONE pin goes low during the V
OUT
trip condition, the
PFET charges the auto-refresh timing node comprised
of R
T
and C
T
, and in turn, pulls the CHRG/IADJ pin low
through a NFET and disables the LT3585 series part. The
DONE pin immediately goes high in shutdown, releasing
the timing node and allowing the voltage at Pins 2 and 3
to decay. After approximately a R
T
C
T
time constant, the
CHRG/IADJ pin is released and the LT3585 series part is
enabled. This cycle is repeated to maintain a constant DC
output voltage. The open-loop control method places a
constraint on the control loop dominant time constant,
R
T
• C
T
, given by:
RC
IL
IV
TT
PK PRI
LK BAT
>
2
2
••
•
where I
LK
is the known leakage current, I
PK
is the trans-
former peak primary current, and L
PRI
is the transformer
primary inductance. If this condition is not met, a runaway
condition could occur. The LT3585 series part would
continue to charge the output voltage past the internal
output trip voltage. Figure 17 shows the AC ripple of a
typical auto-refresh circuit with the proper selection of
R
T
and C
T
.
