LM2651MTC-1.8 National Semiconductor, LM2651MTC-1.8 Datasheet - Page 8

• Manufacturer Part Number: LM2651MTC-1.8
• Description: 1.5A High Efficiency Synchronous Switching Regulator
• Manufacturer: National Semiconductor

www.national.com
Operation
time), the low-side switch is turned on. At the end of the
switching cycle, the low-side switch is turned off; and the
same cycle repeats.
The current of the top switch is sensed by a patented internal
circuitry. This unique technique gets rid of the external sense
resistor, saves cost and size, and improves noise immunity
of the sensed current. A feedforward from the input voltage is
added to reduce the variation of the current limit over the in-
put voltage range.
When the load current decreases below the sleep mode
threshold, the output voltage will rise slightly, this rise is
sensed by the hysteretic mode comparator which makes the
part go into the hysteretic mode with both the high and low
side switches off. The output voltage starts to drop until it hits
the low threshold of the hysteretic comparator, and the part
immediately goes back to the PWM operation. The output
voltage keeps increasing until it reaches the top hysteretic
threshold, then both the high and low side switches turn off
again, and the same cycle repeats.
Protections
The cycle-by-cycle current limit circuitry turns off the
high-side MOSFET whenever the current in MOSFET
reaches 2A.
Design Procedure
This section presents guidelines for selecting external com-
ponents.
INPUT CAPACITOR
A low ESR aluminum, tantalum, or ceramic capacitor is
needed betwen the input pin and power ground. This capaci-
tor prevents large voltage transients from appearing at the
input. The capacitor is selected based on the RMS current
and voltage requirements. The RMS current is given by:
The RMS current reaches its maximum (I
V
the voltage rating should be at least 25% higher than the
maximum input voltage. If a tantalum capacitor is used, the
voltage rating required is about twice the maximum input
voltage. The tantalum capacitor should be surge current
tested by the manufacturer to prevent being shorted by the
inrush current. It is also recommended to put a small ceramic
capacitor (0.1 µF) between the input pin and ground pin to
reduce high frequency spikes.
INDUCTOR
The most critical parameters for the inductor are the induc-
tance, peak current and the DC resistance. The inductance
is related to the peak-to-peak inductor ripple current, the in-
put and the output voltages:
A higher value of ripple current reduces inductance, but in-
creases the conductance loss, core loss, current stress for
the inductor and switch devices. It also requires a bigger out-
put capacitor for the same output voltage ripple requirement.
A reasonable value is setting the ripple current to be 30% of
IN
equals 2V
OUT
(Continued)
. For an aluminum or ceramic capacitor,
OUT
/2) when
8
the DC output current. Since the ripple current increases
with the input voltage, the maximum input voltage is always
used to determine the inductance. The DC resistance of the
inductor is a key parameter for the efficiency. Lower DC re-
sistance is available with a bigger winding area. A good
tradeoff between the efficiency and the core size is letting the
inductor copper loss equal 2% of the output power.
OUTPUT CAPACITOR
The selection of C
output voltage ripple. The output ripple in the constant fre-
quency, PWM mode is approximated by:
The ESR term usually plays the dominant role in determining
the voltage ripple. A low ESR aluminum electrolytic or tanta-
lum capacitor (such as Nichicon PL series, Sanyo OS-CON,
Sprague 593D, 594D, AVX TPS, and CDE polymer alumi-
num) is recommended. An electrolytic capacitor is not rec-
ommended for temperatures below −25˚C since its ESR
rises dramatically at cold temperature. A tantalum capacitor
has a much better ESR specification at cold temperature and
is preferred for low temperature applications.
The output voltage ripple in constant frequency mode has to
be less than the sleep mode voltage hysteresis to avoid en-
tering the sleep mode at full load:
BOOST CAPACITOR
A 0.1 µF ceramic capacitor is recommended for the boost ca-
pacitor. The typical voltage across the boost capacitor is
6.7V.
SOFT-START CAPACITOR
A soft-start capacitor is used to provide the soft-start feature.
When the input voltage is first applied, or when the SD(SS)
pin is allowed to go high, the soft-start capacitor is charged
by a current source (approximately 2 µA). When the SD(SS)
pin voltage reaches 0.6V (shutdown threshold), the internal
regulator circuitry starts to operate. The current charging the
soft-start capacitor increases from 2 µA to approximately
10 µA. With the SD(SS) pin voltage between 0.6V and 1.3V,
the level of the current limit is zero, which means the output
voltage is still zero. When the SD(SS) pin voltage increases
beyond 1.3V, the current limit starts to increase. The switch
duty cycle, which is controlled by the level of the current limit,
starts with narrow pulses and gradually gets wider. At the
same time, the output voltage of the converter increases to-
wards the nominal value, which brings down the output volt-
age of the error amplifier. When the output of the error ampli-
fier is less than the current limit voltage, it takes over the
control of the duty cycle. The converter enters the normal
current-mode PWM operation. The SD(SS) pin voltage is
eventually charged up to about 2V.
The soft-start time can be estimated as:
R
Use the following formula to select the appropriate resistor
values:
where V
1
AND R
T
SS
REF
= C
2
(Programming Output Voltage)
= 1.238V
SS
V
RIPPLE
x 0.6V/2 µA + C
V
OUT
OUT
is driven by the maximum allowable
<
= V
20mV x V
REF
(1 + R
SS
OUT
x (2V−0.6V)/10 µA
1
/R
/V
2
)
FB