Datasheet

LT3500
7
3500fc
TYPICAL PERFORMANCE CHARACTERISTICS
LDRV Dropout Voltage
vs Temperature
Inductor Value for 2A Maximum
Load Current (V
OUT1
= 3.3V,
I
RIPPLE
= 250mA)
V
IN
: The V
IN
pin powers the internal control circuitry and
is monitored by an undervoltage comparator. The V
IN
pin
is also connected to the collectors of the internal power
NPN switch and linear output NPN. The V
IN
pin has high
dI/dt edges and must be decoupled to ground close to
the pin of the device.
SHDN: The SHDN pin is used to shut down the LT3500
and reduce quiescent current to a typical value of
12µA. The accurate 0.76V threshold and input current
hysteresis can be used as an undervoltage lockout, prevent-
ing the regulator from operating until the input voltage has
reached a predetermined level. Force the SHDN pin above
its threshold or let it fl oat for normal operation.
SS: The SS pin is used to control the slew rate of the
output of both the switching and linear regulators. A
single capacitor from the SS pin to ground determines
the regulators’ ramp rate. For soft-start details see the
Applications Information section.
PG: The power good pin is an open-collector output that
sinks current when the FB or LFB falls below 90% of its
nominal regulating voltage. For V
IN
above 2V, its output
state remains true, although during SHDN, V
IN
undervoltage
lockout, or thermal shutdown, its current sink capability
is reduced
V
C
: The V
C
pin is the output of the error amplifi er and the
input to the peak switch current comparator. It is normally
used for frequency compensation, but can also be used
as a current clamp or control loop override. If the error
amplifi er drives V
C
above the maximum switch current
level, a voltage clamp activates. This indicates that the
output is overloaded and current to be pulled from the
SS pin reducing the regulation point.
R
T
/SYNC: This R
T
/SYNC pin provides two modes of setting
the constant switch frequency.
Connecting a resistor from the R
T
/SYNC pin to ground
will set the R
T
/SYNC pin to a typical value of 1V. The
resultant switching frequency will be set by the resistor
value. The minimum value of 15kΩ and maximum value
of 200kΩ set the switching frequency to 2.5MHz and
250kHz respectively.
Switcher Dropout Operation
TEMPERATURE (°C)
–50
1.00
VOLTAGE (V)
1.05
1.15
1.20
1.25
1.50
1.35
0
50
75
3500 G22
1.10
1.40
1.45
1.30
–25 25
100
125
150
I
LDRV
= 5mA
INPUT VOLTAGE (V)
2.5
0
OUTPUT VOLTAGE (V)
1
2
3
4
6
3.0
3.5 4.0 4.5
3500 G23
5.0 5.5
5
I
VOUT1
= 1A
T
A
= 25°C
V
VOUT1
= 5V
V
VOUT1
= 3.3V
INPUT VOLTAGE (V)
5
250
FREQUENCY (kHz)
500
1000
1250
1500
25
2500
3500 G24
750
15
10
30 35
20 40
1750
2000
2250
L = 1.5µH
L = 2.2µH
L = 4.7µH
L = 6.8µH
L = 10µH
L = 3.3µH
L = 1µH
T
A
= 25°C
PIN FUNCTIONS