lm2707 National Semiconductor Corporation, lm2707 Datasheet - Page 11

• Manufacturer Part Number: lm2707
• Description: Inductive-boost Series Led Driver With Programmable Oscillator Frequency
• Manufacturer: National Semiconductor Corporation

Product Description
An example with C
The minimum recommended C
The rise time (t
time (t
The duty cycle of the switching converter (D
slightly greater than the duty cycle of the oscillator because
there is a delay (t
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
V
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:
R
F
. Resistors R
OSC
FIGURE 4. LM2707 Internal Current Limit Circuit
f
). This gives an oscillator duty cycle (D
= 26.3MHz / (15 + 68) = 317kHz.
r
) of the C
1
D
and R
) of approximately 200ns in turning off the
X
= 68pF:
S
2
to a 100mV (typ.) reference voltage
X
provide an input-voltage compo-
signal is 2.2x longer than the fall
X
capacitance is 10pF.
(Continued)
SW
OSC
) is actually
20099230
) of 0.69.
11
An example: when V
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
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:
section)
An example -- V
When the current limit is engaged before the switch is turned
off by the oscillator, the peak inductor current of the circuit
(I
turn-off delay of the power switch:
An example -- V
L-PK-LIMIT
V
L: Inductance
t
I
t
ON
LIMIT
D
IN
: Power transistor turn-off delay (200ns typ.)
: Input voltage (Note 12)
: Switch ON time. (See the Programmable Oscillator
: Current Limit -- 330mA - (V
) is determined by the current limit value and the
S
, R
1
, and R
IN
IN
= 3.0V, L = 22µH, C
= 3.6V, L = 22µH:
IN
= 4.0V, I
2
. This keeps the input current very
LIMIT
IN
) 228mA.
x 25.5mA/V)
X
= 38pF:
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