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參數(shù)資料
| 型號： | LM2716MT-ADJ |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Dual (Step-up and Step-down) PWM DC/DC Converter |
| 中文描述： | DUAL SWITCHING CONTROLLER, 700 kHz SWITCHING FREQ-MAX, PDSO24 |
| 封裝： | TSSOP-24 |
| 文件頁數(shù)： | 9/17頁 |
| 文件大小： | 563K |
| 代理商： | LM2716MT-ADJ |
Buck Operation
PROTECTION (BOTH REGULATORS)
The LM2716 has dedicated protection circuitry running dur-
ing normal operation to protect the IC. The Thermal Shut-
down circuitry turns off the power devices when the die
temperature reaches excessive levels. The UVP comparator
protects the power devices during supply power startup and
shutdown to prevent operation at voltages less than the
minimum input voltage. The OVP comparator is used to
prevent the output voltage from rising at no loads allowing
full PWM operation over all load conditions. The LM2716
also features a shutdown mode for each converter decreas-
ing the supply current to 9μA (both in shutdown mode).
CONTINUOUS CONDUCTION MODE
The LM2716 contains a current-mode, PWM buck regulator.
A buck regulator steps the input voltage down to a lower
output voltage. In continuous conduction mode (when the
inductor current never reaches zero at steady state), the
buck regulator operates in two cycles. The power switch is
connected between V
IN
and SW1.
In the first cycle of operation the transistor is closed and the
diode is reverse biased. Energy is collected in the inductor
and the load current is supplied by C
OUT
and the rising
current through the inductor.
During the second cycle the transistor is open and the diode
is forward biased due to the fact that the inductor current
cannot instantaneously change direction. The energy stored
in the inductor is transferred to the load and output capacitor.
The ratio of these two cycles determines the output voltage.
The output voltage is defined approximately as:
where D is the duty cycle of the switch, D and D' will be
required for design calculations.
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
OUT
/2) when
V
IN
equals 2V
OUT
. This value should be increased by 50% to
account for the ripple current increase due to the boost
regulator. 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.
The minimum capacitor value should be 47μF for lower
output load current applications and less dynamic (quickly
changing) load conditions. For higher output current applica-
tions or dynamic load conditions a 68μF to 100μF low ESR
capacitor is recommended. 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 SELECTION
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.Agood
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
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.
BOOT CAPACITOR
A 3.3 nF ceramic capacitor is recommended for the boot-
strap capacitor.
SOFT-START CAPACITOR (BOTH REGULATORS)
The SS pins are used to tailor the soft-start for a specific
application. A current source charges the external soft-start
capacitor, C
SS
. The soft-start time can be estimated as:
T
SS
= C
SS
*0.6V/I
SS
Soft-start times may be implemented using the SS pin and a
capacitor C
SS
.
When programming the softstart time, simply use the equa-
tion given in the
Soft-Start Capacitor
section above. This
equation uses the typical room temperature value of the soft
start current to set the soft start time.
L
www.national.com
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| LM2716MTX-ADJ | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Dual (Step-up and Step-down) PWM DC/DC Converter |
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