LM2694MTSLASHNOPB datasheet(8/26 Pages) TI

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FS =

VOUT

2 x L1 x 1.54 x 1020

RL x (RON)

DC =

tON

tON + tOFF

VOUT

VIN

= tON x FS

FS =

VOUT x (VIN ± 1.5V)

1.14 x 10

-10 x (R

ON + 1.4 k

:) x VIN

LM2694

SNVS444A – MAY 2006 – REVISED APRIL 2013

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When in regulation, the LM2694 operates in continuous conduction mode at heavy load currents and

discontinuous conduction mode at light load currents. In continuous conduction mode current always flows

through the inductor, never reaching zero during the off-time. In this mode the operating frequency remains

relatively constant with load and line variations. The minimum load current for continuous conduction mode is

one-half the inductor’s ripple current amplitude. The operating frequency is approximately:

(1)

The buck switch duty cycle is equal to:

(2)

In discontinuous conduction mode current through the inductor 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 (C2). 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 losses reduce with

the reduction in load and frequency. The approximate discontinuous operating frequency can be calculated as

follows:

where

•

RL = the load resistance

(3)

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

follows:

VOUT = 2.5 x (R1 + R2) / R2

(4)

Output voltage regulation is based on ripple voltage at the feedback input, requiring a minimum amount of ESR

for the output capacitor C2. The LM2694 requires a minimum of 25 mV of ripple voltage at the FB pin. In cases

where the capacitor’s ESR is insufficient additional series resistance may be required (R3 in Typical Application

Circuit and Block Diagram).

Start-Up Regulator, VCC

The start-up regulator is integral to the LM2694. The input pin (VIN) can be connected directly to line voltage up

to 30V, with transient capability to 33V. The VCC output regulates at 7.0V, and is current limited at 9 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 capacitor (C6) to charge up.

The minimum input voltage is determined by the regulator’s dropout voltage, the VCC UVLO falling threshold

(

≊5.5V), and the frequency. When VCC falls below the falling threshold the VCC UVLO activates to shut off the

output. If VCC is externally loaded, the minimum input voltage increases.

To reduce power dissipation in the start-up regulator, an auxiliary voltage can be diode connected to the VCC pin.

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 (VCC + VIN) cannot exceed 47V. Internally, a

diode connects VCC to VIN. See Figure 10.

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