LT1950
6
1950fa
COMP (Pin 1): The COMP pin is the output of the error
amplifier. The error amplifier is a true op amp which allows
the use of an RC network to be connected between the
Comp and FB pins to compensate the feedback loop for
optimum transient response. The peak switch current in
the external MOSFET will be proportional to the voltage on
the COMP pin. Typical operating voltage range for this pin
is 1V to 2.5V.
FB (Pin 2): The FB pin is the inverting input to the error
amplifier. The output voltage is set with a resistor divider.
The error amplifier adjusts the peak switch current to
maintain the FB pin voltage at the value of the internal
reference voltage of 1.23V.
R
OSC
(Pin 3): A resistor from the R
OSC
pin to ground
programs the operating frequency of the LT1950. Operat-
ing frequency range is 100kHz to 500kHz. Nominal voltage
on the R
OSC
pin is 1V.
SYNC (Pin 4): The SYNC pin is used to synchronize the
internal oscillator to an external clock signal. The pin is
directly logic compatible and can be driven with any signal
with a duty cycle of 10% to 90%. If the SYNC function is
not used the pin can be left open circuit or connected to
ground.
TYPICAL PERFOR A CE CHARACTERISTICS
UW
UU
U
PI FU CTIO S
SLOPE (Pin 5): The SLOPE pin is used to adjust the
amount of slope compensation. Leaving the pin open
circuit results in a default level of slope compensation. The
amount of slope compensation can be adjusted above this
default level by connecting a resistor from the SLOPE pin
to the V
REF
pin.
V
REF
(Pin 6): The V
REF
pin is the output of an internal 2.5V
reference. This pin is capable of sourcing up to 2.5mA for
external use. It is recommended that the V
REF
pin is
bypassed to ground with a 0.1µF ceramic capacitor.
SHDN (Pin 7): The SHDN pin is used to put the device into
a low power shutdown state. In shutdown the V
IN
supply
current drops to 5µA. The SHDN pin has an accurate
threshold of 1.32V which can be used to program an
undervoltage lockout threshold. Input current levels on
the SHDN pin can be used to program hysteresis into the
undervoltage lockout levels.
GND (Pin 8): The GND pin is the analog ground for the
internal circuitry of the LT1950. Sensitive circuitry such as
the feedback divider, frequency setting resistor, reference
bypass capacitor should be tied directly to this pin. See the
Applications Information section for recommendations
on ground connections.
BOOST Switch Off Time vs
Temperature
Maximum Duty Cycle vs
V
SEC
Voltage
GATE Clamp Voltage vs
Temperature
V
SEC
VOLTAGE (V)
MAXIMUM DUTY CYCLE (%)
100
90
80
70
60
50
40
30
1950 G17
0.8
1.2
1.6 2.0 2.4 2.8 3.2
TEMPERATURE (°C)
–50
GATE CLAMP VOLTAGE (V)
100
1950 G18
050
16
15
14
13
12
11
10
–25 25 75 125
MAX DUTY CYCLE = (105/V
SEC
)%
1.25V < V
SEC
< 2.8V
T
A
= 25°C
BOOST SWITCH OFF TIME (ns)
700
600
500
400
300
TEMPERATURE (°C)
–50 25 75
1950 G16
–25 0
50 100 125
