LM2831ZMF EVAL National Semiconductor, LM2831ZMF EVAL Datasheet - Page 11

• Manufacturer Part Number: LM2831ZMF EVAL
• Description: BOARD EVAL LM2831ZMF
• Manufacturer: National Semiconductor

Specifications of LM2831ZMF EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 0.6 ~ 5.5V
• Current - Output: 1.5A
• Voltage - Input: 3 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 3MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2831
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

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 the Simplified Block Diagram (Figure 1) and to the wave-
forms in Figure 2. 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
up to approximately V
creases with a linear slope. I
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 am-
plifier’s output, which is proportional to the difference be-
tween the feedback voltage and V
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 di-
ode, which forces the SW pin to swing below ground by the
forward voltage (V
lator loop adjusts the duty cycle (D) to maintain a constant
output voltage.
SOFT-START
This function forces V
during start up. During soft-start, the error amplifier’s refer-
ence 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.
FIGURE 2. Typical Waveforms
D
) of the Schottky catch diode. The regu-
OUT
IN
, and the inductor current (I
to increase at a controlled rate
L
is measured by the current
REF
. When the PWM
SW
20174866
) swings
L
) in-
11
OUTPUT OVERVOLTAGE PROTECTION
The over-voltage comparator compares the FB pin voltage
to a voltage that is 15% higher than the internal reference
V
reference, the internal PMOS control switch is turned off,
which allows the output voltage to decrease toward regula-
tion.
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
teresis prevents the part from turning off during power up if
V
CURRENT LIMIT
The LM2831 uses cycle-by-cycle current limiting to protect
the output switch. During each switching cycle, a current limit
comparator detects if the output switch current exceeds 2.5A
(typ), and turns off the switch until the next switching cycle
begins.
THERMAL SHUTDOWN
Thermal shutdown limits total power dissipation by turning
off the output switch when the IC junction temperature ex-
ceeds 165˚C. After thermal shutdown occurs, the output
switch doesn’t turn on until the junction temperature drops to
approximately 150˚C.
Design Guide
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the
ratio of output voltage (V
The catch diode (D1) forward voltage drop and the voltage
drop across the internal PMOS must be included to calculate
a more accurate duty cycle. Calculate D by using the follow-
ing formula:
V
The diode forward drop (V
depending on the quality of the diode. The lower the V
higher the operating efficiency of the converter. The inductor
value determines the output ripple current. Lower inductor
values decrease the size of the inductor, but increase the
output ripple current. An increase in the inductor value will
decrease the output ripple current.
One must ensure that the minimum current limit (1.8A) is not
exceeded, so the peak current in the inductor must be
calculated. The peak current (I
lated by:
REF
IN
SW
is non-monotonic.
. Once the FB pin voltage goes 15% above the internal
can be approximated by:
V
SW
I
LPK
= I
O
= I
) to input voltage (V
OUT
D
IN
) can range from 0.3V to 0.7V
OUT
drops below 2.3V (typ). Hys-
LPK
x R
+ ∆i
) in the inductor is calcu-
DSON
L
IN
):
www.national.com
D
, the