LT3505 LINER [Linear Technology], LT3505 Datasheet - Page 20

• Manufacturer Part Number: LT3505
• Description: Monolithic Dual Tracking 3A Step-Down Switching
• Manufacturer: LINER [Linear Technology]

LT3501
applicaTions inForMaTion
If the synchronization signal powers up in a high impedance
state (Hi-Z), connect a resistor from the R
ground. The equivalent resistance seen from the R
pin to ground will set the start-up frequency.
If the synchronization signal changes between high and
low impedance states during power-up (V
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
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
used to prevent excessive discharge of battery-operated
systems.
Since V
not monitored, care must be taken to insure that V
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
20
V
V
IN1
IN1
OR V
> 2.8V
IN2
IN2
C1
supplies the output stage of channel 2 and is
R1
R2
Figure 8. Undervoltage Lockout
LT3501
V
SHDN
IN1
IN1
3µA
of 2.8V. This feature can be
7µA
T
/SYNC pin to ground.
+
1.28V
OL
, Hi-Z), connect
T
/SYNC pin to
–
+
REGULATOR
INTERNAL
T
/SYNC
3501 F08
IN2
is
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:
Example: switching should not start until the input is above
4.75V and is to stop if the input falls below 3.75V.
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:
V
V
V
V
R1=
R2 =
R1=
R2 =
t
RAMP
H
L
H
L
= Turn-off threshold
= 3.75V
= Turn-on threshold
= 4.75V
V
4.75 – 3.75
4.75 – 1.28
V
H
7µA
=
H
– V
– 1.28
7µA
143k
c
R1
SS
3.25µA
L
1.28
1.28
• 0.8V
+ 3µA
≅ 143k
+ 3µA
≅ 47k
3501fd