Start-Up Regulator, V

CC

The start-up regulator is integral to the LM2695. The input

pin (VIN) can be connected directly to line voltage up to 30V,

with transient capability to 33V. The V

7.0V, and is current limited at 9.7 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 capaci-

tor (C6) to charge up.

The minimum input voltage is determined by the regulator’s

dropout voltage, the V

)5.5V),

and the frequency. When V

old the V

externally loaded, the minimum input voltage increases

since the output impedance at V

)140

Ω.

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 VIN. See Figure 5.

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

programmed 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. Input bias current at the FB

pin is less than 100 nA over temperature.

Over-Voltage Comparator

The voltage at FB is compared to an internal 2.9V reference.

If the voltage at FB rises above 2.9V the on-time pulse is

immediately terminated. This condition can occur if the input

voltage or the output load changes suddenly, or if the induc-

tor (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 LM2695 is determined by the R

tor and the input voltage (V

(4)

See Figure 2. The inverse relationship with V

nearly constant frequency as V

continuous conduction mode switching frequency (FS), the

R

(5)

In high frequency applicatons the minimum value for t

limited by the maximum duty cycle required for regulation

off-time limits the maximum duty cycle achievable with a low

voltage at V

desired V

following:

(6)

20170411

FIGURE 5. Self Biased Configuration

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