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
LM2832
www.ti.com SNVS455A –AUGUST 2006–REVISED APRIL 2013
APPLICATIONS INFORMATION
THEORY OF OPERATION
The LM2832 is a constant frequency PWM buck regulator IC that delivers a 2.0A load current. The regulator has
a preset switching frequency of 1.6MHz or 3.0MHz. This high frequency allows the LM2832 to operate with small
surface mount capacitors and inductors, resulting in a DC/DC converter that requires a minimum amount of
board space. The LM2832 is internally compensated, so it is simple to use and requires few external
components. The LM2832 uses current-mode control to regulate the output voltage. The following operating
description of the LM2832 will refer to the Simplified Block Diagram (Figure 23) and to the waveforms in
Figure 24. The LM2832 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 LM2832 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.
Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 9
Product Folder Links: LM2832