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参数资料
| 型号： | LM2651MTC-2.5/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 2.6 A SWITCHING REGULATOR, 345 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封装： | TSSOP-16 |
| 文件页数： | 11/11页 |
| 文件大小： | 631K |
| 代理商： | LM2651MTC-2.5/NOPB |
Design Procedure (Continued)
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
OUT/2)
when
V
IN equals 2VOUT. For an aluminum or ceramic capacitor,
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 F) 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
input and the output voltages:
A higher value of ripple current reduces inductance, but
increases the conductance loss, core loss, current stress for
the inductor and switch devices. It also requires a bigger
output capacitor for the same output voltage ripple require-
ment. A reasonable value is setting the ripple current to be
30% of the DC output current. Since the ripple current in-
creases 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 resistance 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
OUT is driven by the maximum allowable
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 25C 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
entering the sleep mode at full load:
V
RIPPLE < 20mV x VOUT /VFB
BOOST CAPACITOR
A 0.1 F ceramic capacitor is recommended for the boost
capacitor. 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 A). 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 A to approximately
10 A. 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
towards the nominal value, which brings down the output
voltage of the error amplifier. When the output of the error
amplifier 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:
T
SS =CSS x 0.6V/2 A + CSS x (2V0.6V)/10 A
R
1 AND R2 (Programming Output Voltage)
Use the following formula to select the appropriate resistor
values:
V
OUT =VREF(1+R1/R2)
where V
REF = 1.238V
Select resistors between 10k
and 100k. (1% or higher
accuracy metal film resistors for R
1 and R2.)
COMPENSATION COMPONENTS
In the control to output transfer function, the first pole F
p1 can
be estimated as 1/(2
πR
OUTCOUT); The ESR zero Fz1 of the
output capacitor is 1/(2
πESRC
OUT); Also, there is a high
frequency pole F
p2 in the range of 45kHz to 150kHz:
F
p2 =Fs/(
πn(1D))
whereD=V
OUT/VIN, n = 1+0.348L/(VINVOUT)(L isinHs
and V
IN and VOUT in volts).
The total loop gain G is approximately 500/I
OUT where IOUT
is in amperes.
A Gm amplifier is used inside the LM2651. The output resis-
tor R
o of the Gm amplifier is about 80k
.C
c1 and RC
together with R
o give a lag compensation to roll off the gain:
F
pc1 = 1/(2
πC
c1(Ro+Rc)), Fzc1 = 1/2
πC
c1Rc.
In some applications, the ESR zero F
z1 can not be cancelled
by F
p2. Then, Cc2 is needed to introduce Fpc2 to cancel the
ESR zero, F
p2 = 1/(2
πC
c2Ro\Rc).
The rule of thumb is to have more than 45 phase margin at
the crossover frequency (G=1).
If C
OUT is higher than 68F, Cc1 = 2.2nF, and Rc = 15K
are
good choices for most applications. If the ESR zero is too
low to be cancelled by F
p2, add Cc2.
If the transient response to a step load is important, choose
R
C to be higher than 10k
.
LM2651
www.national.com
9
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