LT3500 LINER [Linear Technology], LT3500 Datasheet - Page 20

• Manufacturer Part Number: LT3500
• Description: Monolithic 2A Step-Down Regulator Plus Linear Regulator/Controller
• Manufacturer: LINER [Linear Technology]

LT3500
If the synchronization signal changes between high and
low impedance states during power up (V
the synchronization circuitry to the LT3500 as shown in
the Typical Applications section. This will allow the LT3500
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 an undervoltage lockout (UVLO)
to the LT3500. 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.
If an adjustable UVLO threshold is required, the SHDN
pin can be used. The threshold voltage of the SHDN pin
comparator is 0.76V. A 2.5μA internal current source de-
faults the open-pin condition to be operating (see Typical
Performance Characteristics). Current hysteresis is added
APPLICATIONS INFORMATION
20
C1
Figure 8. Undervoltage Lockout
R1
R2
SHDN
IN
V
IN
of 2.8V. This feature can be
0.76V
T
2.5μA
/SYNC pin to ground.
2μA
+
–
OL
, Hi-Z), connect
3500 F08
above the SHDN threshold. This can be used to set voltage
hysteresis of the UVLO using the following:
V
V
Example: switching should not start until the input is above
4.75V and is to stop if the input falls below 3.75V.
V
V
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 outputs of the LT3500 regulate to either the SS pin
voltage minus 100mV or an internally regulated 800mV,
whichever is lowest. A capacitor from the SS pin to ground
is charged by an internal 2.75μA current source resulting
in a linear output ramp from 0V to the regulated output
whose duration is given by:
H
L
H
L
R1=
R2 =
R1=
R2 =
= Turn-off threshold
= 3.75
t
= Turn-on threshold
= 4.75V
RAMP
V
4.75 − 3.75
V
4.75 − 0.76
H
2μA
=
H
− V
2μA
− 0.76
499k
C
R1
SS
2.75μA
L
0.76
0.76
• 0.9V
+ 2.5μA
~ 499k
+ 2.5μA
~ 71.5k
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