Datasheet

F
S
=
V
OUT
2
x L1 x 1.54 x 10
20
R
L
x (R
ON
)
2
DC =
t
ON
t
ON
+ t
OFF
=
V
OUT
V
IN
= t
ON
x F
S
F
S
=
V
OUT
x (V
IN
± 1.5V)
1.14 x 10
-10
x (R
ON
+ 1.4 k:) x V
IN
LM2694
SNVS444A –MAY 2006–REVISED APRIL 2013 www.ti.com
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
• R
L
= the load resistance (3)
The output voltage is set by two external resistors (R1, R2). The regulated output voltage is calculated as
follows:
V
OUT
= 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, V
CC
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 V
CC
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 V
CC
UVLO falling threshold
(≊5.5V), and the frequency. When V
CC
falls below the falling threshold the V
CC
UVLO activates to shut off the
output. If V
CC
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 V
CC
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 (V
CC
+ V
IN
) cannot exceed 47V. Internally, a
diode connects VCC to VIN. See Figure 10.
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