LM2830XMF National Semiconductor, LM2830XMF Datasheet - Page 10

• Manufacturer Part Number: LM2830XMF
• Description: BUCK REG 1A 1.6MHZ, SMD, SOT-23-5
• Manufacturer: National Semiconductor

Specifications of LM2830XMF

• Primary Input Voltage: 5.5V
• No. Of Outputs: 1
• Output Voltage: 4.5V
• Output Current: 1A
• Voltage Regulator Case Style: SOT-23
• No. Of Pins: 5
• Operating Temperature Range: -40°C To +125°C
• Svhc: No SVHC
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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Applications Information
THEORY OF OPERATION
The LM2830 is a constant frequency PWM buck regulator IC
that delivers a 1.0A load current. The regulator has a preset
switching frequency of 1.6MHz or 3.0MHz. This high fre-
quency allows the LM2830 to operate with small surface
mount capacitors and inductors, resulting in a DC/DC con-
verter that requires a minimum amount of board space. The
LM2830 is internally compensated, so it is simple to use and
requires few external components. The LM2830 uses
current-mode control to regulate the output voltage. The
following operating description of the LM2830 will refer to the
Simplified Block Diagram (Figure 1) and to the waveforms in
Figure 2. The LM2830 supplies a regulated output voltage by
switching the internal PMOS control switch at constant fre-
quency 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
V
slope. I
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 pro-
portional to the difference between the feedback voltage and
V
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
Schottky catch diode. The regulator 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.
IN
REF
, and the inductor current (I
. When the PWM comparator output goes high, the
L
is measured by the current sense amplifier, which
FIGURE 2. Typical Waveforms
OUT
to increase at a controlled rate
SW
) swings up to approximately
L
) increases with a linear
D
20197466
) of the
10
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 LM2830 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 LM2830 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
1.75A (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 (1.2A) 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
):
D
, the