Start-up Regulator, V

CC

The start-up regulator is integral to the LM34910. The input

pin (V

with transient capability to 40V. The V

7.0V, and is current limited to 9 mA. Upon power up, the

regulator sources current into the external capacitor at V

CC

(C3). When the voltage on the V

voltage lockout threshold of 5.8V, the buck switch is enabled

and the Softstart pin is released to allow the Softstart capaci-

tor (C6) to charge up.

The minimum input voltage is determined by the regulator’s

dropout voltage, the V

)5.7V),

and the frequency. When V

old the V

externally loaded, the minimum input voltage increases

since the output impedance at V

)140

Ω. See Figure 2.

To reduce power dissipation in the start-up regulator, an

auxiliary voltage can be diode connected to the V

Setting the auxiliary voltage to between 8V and 14V shuts off

the internal regulator, reducing internal power dissipation.

The sum of the auxiliary voltage and the input voltage (V

CC

+V

to V

Regulation Comparator

The feedback voltage at FB is compared to the voltage at the

Softstart pin (2.5V). In normal operation (the output voltage

is regulated), an on-time period is initiated when the voltage

at FB falls below 2.5V. The buck switch stays on for the

on-time, causing the FB voltage to rise above 2.5V. After the

on-time period, the buck switch stays off until the FB voltage

falls below 2.5V. Bias current at the FB pin is nominally 100

nA.

Over-Voltage Comparator

The voltage at FB is compared to an internal 2.875V refer-

ence. If the voltage at FB rises above 2.875V the on-time

pulse is immediately terminated. This condition can occur if

the input voltage or the output load changes suddenly, or if

the inductor (L1) saturates. The buck switch remains off until

the voltage at FB falls below 2.5V.

ON-Time Timer, and Shutdown

The on-time for the LM34910 is determined by the R

ON

resistor and the input voltage (V

(4)

See Figure 3. The inverse relationship with V

nearly constant frequency as V

selected for a minimum on-time (at maximum V

than 200 ns. This requirement limits the maximum frequency

for each application, depending on V

from the following:

(5)

The LM34910 can be remotely shut down by taking the

R

pin is internally grounded, the on-timer is disabled, and bias

currents are reduced. Releasing the R

mal operation to resume. The voltage at the R

between 1.5V and 3.0V, depending on V

resistor.

20110911

FIGURE 5. Self Biased Configuration

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