Datasheet

R
ON
=
V
OUT
x (V
IN
- 1.5)
F
S
x 1.15 x 10
-10
x V
IN
- 1.4k
t
ON
=
(V
IN
- 1.5)
1.15 x 10
-10
x (R
ON
+ 1.4k)
+ 50 ns
F
S
=
V
OUT
2
x L1 x 1.5 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
± 1.5)
1.15 x 10
-10
x (R
ON
+ 1.4k) x V
IN
LM34914
www.ti.com SNVS453B –MAY 2006–REVISED MARCH 2013
When in regulation, the LM34914 operates in continuous conduction mode at heavy load currents and
discontinuous conduction mode at light load currents. In continuous conduction mode the inductor’s current is
always greater than zero, and 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 approximate operating frequency is calculated as follows:
(1)
The buck switch duty cycle is equal to:
(2)
In discontinuous conduction mode, where the inductor’s current reaches zero during the off-time forcing a longer-
than-normal off-time, 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 decrease with the reduction
in load and frequency. The approximate discontinuous operating frequency can be calculated as follows:
(3)
where R
L
= the load resistance, and L1 is the circuit’s inductor.
The output voltage is set by the two feedback resistors (R1, R2 in the Block Diagram). The regulated output
voltage is calculated as follows:
V
OUT
= 2.5 x (R1 + R2) / R2 (4)
Output voltage regulation is based on supplying ripple voltage to the feedback input (FB pin), normally obtained
from the output voltage ripple through the feedback resistors. The LM34914 requires a minimum of 25 mVp-p of
ripple voltage at the FB pin, requiring the ripple voltage at V
OUT
be higher by the gain factor of the feedback
resistor ratio. The output ripple voltage is created by the inductor’s ripple current passing through R3 which is in
series with the output capacitor. For applications where reduced ripple is required at V
OUT
, see Applications
Information.
If the voltage at FB rises above 2.9V, due to a transient at V
OUT
or excessive inductor current which creates
higher than normal ripple at V
OUT
, the internal over-voltage comparator immediately shuts off the internal buck
switch. The next on-time starts when the voltage FB falls below 2.5V and the inductor current falls below the
current limit threshold.
ON-Time Timer
The on-time for the LM34914 is determined by the R
ON
resistor and the input voltage (V
IN
), calculated from:
(5)
The inverse relationship with V
IN
results in a nearly constant frequency as V
IN
is varied. To set a specific
continuous conduction mode switching frequency (F
S
), the R
ON
resistor is determined from the following:
(6)
Equation 1, Equation 5, and Equation 6 are valid only during normal operation - i.e., the circuit is not in current
limit. When the LM34914 operates in current limit, the on-time is reduced by approximately 50%. This feature
reduces the peak inductor current which may be excessively high if the load current and the input voltage are
simultaneously high. This feature operates on a cycle-by-cycle basis until the load current is reduced and the
output voltage resumes its normal regulated value.
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