Datasheet

LT3501
20
3501fd
Figure 8. Undervoltage Lockout
If the synchronization signal powers up in a high impedance
state (Hi-Z), connect a resistor from the R
T
/SYNC pin to
ground. The equivalent resistance seen from the R
T
/SYNC
pin to ground will set the start-up frequency.
If the synchronization signal changes between high and
low impedance states during power-up (V
OL
, Hi-Z), connect
the synchronization circuitry to the LT3501 as shown in
the Typical Applications section. This will allow the LT3501
to start-up with a switching frequency determined by the
equivalent resistance from the R
T
/SYNC pin to ground.
Shutdown and Undervoltage Lockout
Figure 8 shows how to add undervoltage lockout (UVLO)
to the LT3501. Typically, UVLO is used in situations where
the input supply is current limited, or has a relatively high
source resistance. A switching regulator draws constant
power from the source, so source current increases as
source voltage drops. This looks like a negative resistance
load to the source and can cause the source to current limit
or latch low under low source voltage conditions. UVLO
prevents the regulator from operating at source voltages
where these problems might occur.
An internal comparator will force the part into shutdown
below the minimum V
IN1
of 2.8V. This feature can be
used to prevent excessive discharge of battery-operated
systems.
Since V
IN2
supplies the output stage of channel 2 and is
not monitored, care must be taken to insure that V
IN2
is
present before channel 2 is allowed to switch.
If an adjustable UVLO threshold is required, the SHDN
pin can be used. The threshold voltage of the SHDN
pin comparator is 1.28V. A 3µA internal current source
defaults the open-pin condition to be operating (see Typical
Performance Characteristics). Current hysteresis is added
above the SHDN threshold. This can be used to set voltage
hysteresis of the UVLO using the following:
R1=
V
H
– V
L
7µA
R2 =
1.28
V
H
– 1.28
R1
+ 3µA
V
H
= Turn-on threshold
V
L
= Turn-off threshold
Example: switching should not start until the input is above
4.75V and is to stop if the input falls below 3.75V.
V
H
= 4.75V
V
L
= 3.75V
R1=
4.75 – 3.75
7µA
≅ 143k
R2 =
1.28
4.75 – 1.28
143k
+ 3µA
≅ 47k
Keep the connections from the resistors to the SHDN
pin short and make sure that the interplane or surface
capacitance to switching nodes is minimized. If high re-
sistor values are used, the SHDN pin should be bypassed
with a 1nF capacitor to prevent coupling problems from
the switch node.
Soft-Start
The output of the LT3501 regulates to the lowest voltage
present at either the SS pin or an internal 0.8V reference.
A capacitor from the SS pin to ground is charged by an
internal 3.25µA current source resulting in a linear output
ramp from 0V to the regulated output whose duration is
given by:
t
RAMP
=
C
SS
• 0.8V
3.25µA
+
+
–
1.28V
7µA
3µA
R1
R2C1
SHDN
INTERNAL
REGULATOR
V
IN1
V
IN1
> 2.8V
3501 F08
V
IN1
OR V
IN2
LT3501
applicaTions inForMaTion