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參數(shù)資料
| 型號(hào)： | LM2707MF |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 顯示驅(qū)動(dòng)器 |
| 英文描述： | Inductive-Boost Series LED Driver with Programmable Oscillator Frequency |
| 中文描述： | LED DISPLAY DRIVER, PDSO8 |
| 封裝： | SOT-23, 8 PIN |
| 文件頁(yè)數(shù)： | 11/19頁(yè) |
| 文件大小： | 1039K |
| 代理商： | LM2707MF |
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Product Description
(Continued)
An example with C
X
= 68pF:
F
OSC
= 26.3MHz / (15 + 68) = 317kHz.
The minimum recommended C
X
capacitance is 10pF.
The rise time (t
) of the C
signal is 2.2x longer than the fall
time (t
). This gives an oscillator duty cycle (D
OSC
) of 0.69.
The duty cycle of the switching converter (D
slightly greater than the duty cycle of the oscillator because
there is a delay (t
D
) of approximately 200ns in turning off the
power transistor.
The output of the oscillator connects to the R-S latch. When
the reset signal of the latch is low, the oscillator signal
becomes the ON/OFF signal for the power transistor, as
described in the previous section.
CURRENT LIMIT
The LM2707 current limit circuit senses the current through
the inductor and interrupts switching when the current limit
threshold is exceeded. The current limit circuit is shown in
Figure 4
.
The current limit circuit operates by comparing the voltage
across sense resistor R
S
to a 100mV (typ.) reference voltage
V
R
. Resistors R
1
and R
2
provide an input-voltage compo-
nent to the current limit that causes the current limit to be
lower at higher input voltages.
The current limit threshold can be calculated by determining
when the voltages on the current limit comparator inputs are
equal:
An example: when V
IN
= 4.0V, I
LIMIT
)
228mA.
When the current limit comparator trips, the comparator
output causes the R-S latch to reset, and the power transis-
tor is turned off. The transistor does not turn off immediately,
though. There is a 200ns (typ.) delay between when the
comparator trips and the power transistor turns off. Because
of this delay, the peak inductor current rises above the
current limit threshold. Peak inductor current is discussed
and calculated in the section to follow:
Peak Inductor Cur-
rent
.
The transistor Q1 in
Figure 4
opens when the EN signal is
low, blocking the current path from input to ground through
resistors R
, R
, and R
2
. This keeps the input current very
low during shutdown.
PEAK INDUCTOR CURRENT
When conditions exist such that current limit is not reached
during normal steady-state operation, peak inductor current
is determined by the power switch ON time and can be
predicted with the following equation:
V
IN
: Input voltage (Note 12)
L: Inductance
t
: Switch ON time. (See the
Programmable Oscillator
section)
An example -- V
IN
= 3.0V, L = 22μH, C
X
= 38pF:
When the current limit is engaged before the switch is turned
off by the oscillator, the peak inductor current of the circuit
(I
) is determined by the current limit value and the
turn-off delay of the power switch:
I
LIMIT
: Current Limit -- 330mA - (V
IN
x 25.5mA/V)
t
D
: Power transistor turn-off delay (200ns typ.)
An example -- V
IN
= 3.6V, L = 22μH:
20099230
FIGURE 4. LM2707 Internal Current Limit Circuit
L
www.national.com
11
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