tion mode is one-half the inductor’s ripple current amplitude.

The operating frequency is approximately:

(1)

The buck switch duty cycle is approximately equal to:

(2)

In discontinuous conduction mode current through the induc-

tor ramps up from zero to a peak during the on-time, then

ramps back to zero before the end of the off-time. The next

on-time period starts when the voltage at FB falls below the

reference - until then the inductor current remains zero, and

the load current is supplied by the output capacitor. In this

mode the operating frequency is lower than in continuous

conduction mode, and varies with load current. Conversion

efficiency is maintained at light loads since the switching loss-

es decrease with the reduction in load and frequency. The

approximate discontinuous operating frequency can be cal-

culated as follows:

(3)

where R

The output voltage is set by two external resistors (R1, R2).

The regulated output voltage is calculated as follows:

V

Output voltage regulation is based on ripple voltage at the

feedback input,normally obtained from the output voltage rip-

ple through the feedback resistors. The LM34919 requires a

minimum of 25 mV of ripple voltage at the FB pin. In cases

where the capacitor’s ESR is insufficient additional series re-

sistance may be required (R3).

Start-Up Regulator, V

CC

The start-up regulator is integral to the LM34919. The input

pin (VIN) can be connected directly to line voltage up to 40V,

with transient capability to 44V. The V

7.0V, and is current limited at 9.5 mA. Upon power up, the

regulator sources current into the external capacitor at VCC

(C3). When the voltage on the VCC pin reaches the under-

voltage lockout threshold of 5.7V, the buck switch is enabled

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

itor (C6) to charge up.

The minimum input voltage is determined by the regulator’s

dropout voltage, the V

and the frequency. When V

the V

nally loaded, the minimum input voltage increases.

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

iliary voltage can be diode connected to the V

the auxiliary voltage to between 7V and 14V shuts off the in-

ternal regulator, reducing internal power dissipation. The sum

of the auxiliary voltage and the input voltage (V

not exceed 52V. Internally, a diode connects VCC to VIN. See

Figure 2.

30004411

FIGURE 2. Self Biased Configuration

9

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