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參數(shù)資料
| 型號： | LM2702MT-ADJ/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 2.6 A SWITCHING REGULATOR, 720 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | TSSOP-16 |
| 文件頁數(shù)： | 8/21頁 |
| 文件大?。?/td> | 1197K |
| 代理商： | LM2702MT-ADJ/NOPB |
Operation (Continued)
section, the soft start curent has a temperature coefficient
and would lead one to believe there would be significant
variation with temperature. Though the current has a tem-
perature coefficient the actual programmed external soft
start time does not show this extreme of a temperature
variation. As you can see in the following transient plots:
V
OUT =8V, VIN = 2.5V, RL =51
,C
SS = 330nF, T = 4ms/div.
Trace:
1) V
IN, 5V/div, DC Coupled
2) V
OUT, 5V/div, DC Coupled
3) I
L, 0.5A/div, DC Coupled
4) V
SW, 5V/div, DC Coupled
20051169
T
A = 20C
20051170
T
A = 27C
20051171
T
A = 85C
When programming the softstart time externally, simply use
the equation given in the Soft-Start Capacitor section above.
This equation uses the typical room temperature value of the
soft start current, 12A, to set the soft start time.
DELAY CAPACITOR
The LM2702 has internal circuitry that can be used to set a
delay time preventing control of the PMOS switch via SWC
until a desired amount of time after the switcher starts up.
The PMOS control circuitry remains inactive until V
OUT
reaches 85% of the nominal output voltage. When this oc-
curs, C
D begins to charge. When the voltage on the Delay
pin reaches 1.265V the PMOS switch will become active and
can be controlled using the SWC pin. If no C
D is used, the
PMOS switch can be controlled immediately after V
OUT
reaches 85% of the nominal output voltage. The delay time
can be calculated using the equation:
T
D =CD * (1.265V/5.7A)
INTRODUCTION TO COMPENSATION
The LM2702 contains a current mode PWM boost converter.
The signal flow of this control scheme has two feedback
loops, one that senses switch current and one that senses
output voltage.
To keep a current programmed control converter stable
above duty cycles of 50%, the inductor must meet certain
criteria. The inductor, along with input and output voltage,
will determine the slope of the current through the inductor
(see Figure 2 (a)). If the slope of the inductor current is too
great, the circuit will be unstable above duty cycles of 50%.
A 4.7H inductor is recommended for most applications. If
the duty cycle is approaching the maximum of 85%, it may
be necessary to increase the inductance by as much as 2X.
See Inductor and Diode Selection for more detailed inductor
sizing.
The LM2702 provides a compensation pin (V
C) to customize
the voltage loop feedback. It is recommended that a series
combination of R
C and CC be used for the compensation
network, as shown in the typical application circuit. For any
given application, there exists a unique combination of R
C
and C
C that will optimize the performance of the LM2702
circuit in terms of its transient response. The series combi-
nation of R
C and CC introduces a pole-zero pair according to
the following equations:
20051105
FIGURE 2. (a) Inductor current. (b) Diode current.
LM2702
www.national.com
16
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