4
LT3436
3436fa
TYPICAL PERFOR A CE CHARACTERISTICS
UW
GND (Pins 1, 5, 6, 8, 9, 16, 17): Short GND pins 1, 5, 6,8,
9, 16 and the exposed pad (pin 17) on the PCB. The GND
is the reference for the regulated output, so load regulation
will suffer if the “ground” end of the load is not at the same
voltage as the GND of the IC. This condition occurs when
the load current flows through the metal path between the
GND pins and the load ground point. Keep the ground path
short between the GND pins and the load and use a ground
plane when possible. Keep the path between the input
bypass and the GND pins short. The exposed pad should
be attached to a large copper area to improve thermal
performance.
V
IN
(Pin 2): This pin powers the internal circuitry and
internal regulator. Keep the external bypass capacitor
close to this pin.
SW (Pins 3, 4): The switch pin is the collector of the on-
chip power NPN switch and has large currents flowing
through it. Keep the traces to the switching components as
short as possible to minimize radiation and voltage spikes.
SHDN (Pin 11): The shutdown pin is used to turn off the
regulator and to reduce input drain current to a few
microamperes. The 1.35V threshold can function as an
accurate undervoltage lockout (UVLO), preventing the
regulator from operating until the input voltage has reached
a predetermined level. Float or pull high to put the regula-
tor in the operating mode.
FB (Pin 12): The feedback pin is used to set output voltage
using an external voltage divider that generates 1.2V at the
pin with the desired output voltage. If required, the current
limit can be reduced during start up when the FB pin is
below 0.5V (see the Current Limit Foldback graph in the
Typical Performance Characteristics section). An imped-
ance of less than 5kΩ at the FB pin is needed for this
feature to operate.
V
C
(Pin 13): The V
C
pin is the output of the error amplifier
and the input of the peak switch current comparator. It is
normally used for frequency compensation, but can do
double duty as a current clamp or control loop override.
This pin sits at about 0.3V for very light loads and 0.9V at
maximum load.
SYNC (Pin 14): The sync pin is used to synchronize the
internal oscillator to an external signal. It is directly logic
compatible and can be driven with any signal between
20% and 80% duty cycle. The synchronizing range is
equal to
initial
operating frequency, up to 1.4MHz. See
Synchronization section in Applications Information for
details. When not in use, this pin should be grounded.
SHDN Supply Current Input Supply Current
Current Limit Foldback
PIN FUNCTIONS
UUU
SHDN VOLTAGE (V)
0
V
IN
CURRENT (µA)
300
250
200
150
100
50
0
0.6 1.0
3436 G07
0.2 0.4
0.8 1.2 1.4
INPUT VOLTAGE (V)
0
V
IN
CURRENT (µA)
1200
1000
800
600
400
200
0
5
10 15 20
3436 G08
25 30
FEEDBACK VOLTAGE (V)
0 0.2
SWITCH PEAK CURRENT (A)
0.4 0.80.6
1.0
1.2
3436 G09
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
FB INPUT CURRENT (µA)
40
30
20
10
0
T
A
= 25°C
V
IN
= 15V
T
A
= 25°C
T
A
= 25°C
MINIMUM
INPUT
VOLTAGE
SWITCH CURRENT
