Datasheet

0
0
V
IN
V
D
T
ON
t
t
Inductor
Current
D = T
ON
/T
SW
V
SW
T
OFF
T
SW
I
L
I
PK
SW
Voltage
LM2831
SNVS422C –AUGUST 2006–REVISED APRIL 2013 www.ti.com
APPLICATIONS INFORMATION
THEORY OF OPERATION
The LM2831 is a constant frequency PWM buck regulator IC that delivers a 1.5A load current. The regulator has
a preset switching frequency of 550kHz, 1.6MHz, or 3.0MHz. This high frequency allows the LM2831 to operate
with small surface mount capacitors and inductors, resulting in a DC/DC converter that requires a minimum
amount of board space. The LM2831 is internally compensated, so it is simple to use and requires few external
components. The LM2831 uses current-mode control to regulate the output voltage. The following operating
description of the LM2831 will refer to Figure 23 and to the waveforms in Figure 24. The LM2831 supplies a
regulated output voltage by switching the internal PMOS control switch at constant frequency and variable duty
cycle. A switching cycle begins at the falling edge of the reset pulse generated by the internal oscillator. When
this pulse goes low, the output control logic turns on the internal PMOS control switch. During this on-time, the
SW pin voltage (V
SW
) swings up to approximately V
IN
, and the inductor current (I
L
) increases with a linear slope.
I
L
is measured by the current sense amplifier, which generates an output proportional to the switch current. The
sense signal is summed with the regulator’s corrective ramp and compared to the error amplifier’s output, which
is proportional to the difference between the feedback voltage and V
REF
. When the PWM comparator output goes
high, the output switch turns off until the next switching cycle begins. During the switch off-time, inductor current
discharges through the Schottky catch diode, which forces the SW pin to swing below ground by the forward
voltage (V
D
) of the Schottky catch diode. The regulator loop adjusts the duty cycle (D) to maintain a constant
output voltage.
Figure 24. Typical Waveforms
SOFT-START
This function forces V
OUT
to increase at a controlled rate during start up. During soft-start, the error amplifier’s
reference voltage ramps from 0V to its nominal value of 0.6V in approximately 600 µs. This forces the regulator
output to ramp up in a controlled fashion, which helps reduce inrush current.
OUTPUT OVERVOLTAGE PROTECTION
The over-voltage comparator compares the FB pin voltage to a voltage that is 15% higher than the internal
reference V
REF
. Once the FB pin voltage goes 15% above the internal reference, the internal PMOS control
switch is turned off, which allows the output voltage to decrease toward regulation.
UNDERVOLTAGE LOCKOUT
Under-voltage lockout (UVLO) prevents the LM2831 from operating until the input voltage exceeds 2.73V (typ).
The UVLO threshold has approximately 430 mV of hysteresis, so the part will operate until V
IN
drops below 2.3V
(typ). Hysteresis prevents the part from turning off during power up if V
IN
is non-monotonic.
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