LM2707 datasheet(11/19 Pages) NSC

11 / 19 page

Product Description (Continued)

An example with C

X = 68pF:

OSC = 26.3MHz / (15 + 68) = 317kHz.

The minimum recommended C

X capacitance is 10pF.

The rise time (t

r) of the CX signal is 2.2x longer than the fall

time (t

f). This gives an oscillator duty cycle (DOSC) of 0.69.

The duty cycle of the switching converter (D

SW) is actually

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

R. Resistors R1 and R2 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, ILIMIT

) 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

S,R1, and R2. 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:

IN: Input voltage (Note 12)

L: Inductance

ON: Switch ON time. (See the Programmable Oscillator

section)

An example -- V

IN = 3.0V, L = 22µH, CX = 38pF:

When the current limit is engaged before the switch is turned

off by the oscillator, the peak inductor current of the circuit

L-PK-LIMIT) is determined by the current limit value and the

turn-off delay of the power switch:

LIMIT: Current Limit -- 330mA - (VIN x 25.5mA/V)

D: Power transistor turn-off delay (200ns typ.)

An example -- V

IN = 3.6V, L = 22µH:

20099230

FIGURE 4. LM2707 Internal Current Limit Circuit

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