LM2833 Datasheet, PDF (15/30 Pages) National Semiconductor (TI) – 1.5MHz/3MHz 3.0A Step-Down DC-DC Switching Regulator

LM2833
www.ti.com
SNVS505E – MAY 2008 – REVISED APRIL 2013
When the designed maximum output current is reduced, the ratio r can be increased. At a current of 0.1A, r can
be made as high as 0.9. The ripple ratio can be increased at lighter loads because the net ripple is actually quite
low, and if r remains constant the inductor value can be made quite large. An equation empirically developed for
the maximum ripple ratio at any current below 2A is:
r = 0.387 x I_OUT-0.3667
(8)
Note that this is just a guideline, and it needs to be combined with two important factors for proper selection of
inductance values at any operating condition. The first consideration is at output voltage above 2.5V, one needs
to ensure that the inductance given by the above guideline should not be less than 1µH for the LM2833X or
0.5µH for the LM2833Z. Since the LM2833 has a fixed internal corrective ramp signal, a very low inductance
value at high output voltage will generate a very steep down slope of inductor current, which will result in an
insufficient slope compensation, and cause instability known as sub-harmonic oscillation. Another consideration
is at low load current, one needs to ensure that the inductance value given by the guideline should not exceed
10µH for the LM2833X and 4.7µH for the LM2833Z, since too much inductance effectively flattens the down
slope of the inductor current, and may significantly limit the system bandwidth and phase margin resulting in
instability.
The LM2833 operates at frequencies allowing the use of ceramic output capacitors without compromising
transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple.
See the OUTPUT CAPACITOR section for more details on calculating output voltage ripple.
Now that the ripple current is determined, the inductance is calculated by:
V_OUT + V_D
L=
x (1-D)
I_OUT x r x f_SW
where
• f_SW is the switching frequency.
(9)
When selecting an inductor, make sure that it is capable of supporting the peak output current without saturating.
Inductor saturation will result in a sudden reduction in inductance and prevent the regulator from operating
properly. Because of the operating frequency of the LM2833, ferrite based inductors are preferred to minimize
core losses. This presents little restriction since the variety and availability of ferrite-based inductors is large.
Lastly, inductors with lower series resistance (DCR) will provide better operating efficiency. For recommended
inductor selection, refer to Design Examples.
INPUT CAPACITOR
An input capacitor is necessary to ensure that V_IN does not drop excessively during switching transients. The
primary specifications of the input capacitor are capacitance, voltage rating, RMS current rating, and ESL
(Equivalent Series Inductance). The input voltage rating is specifically stated by the capacitor manufacturer.
Make sure to check any recommended deratings and also verify if there is any significant change in capacitance
at the operating input voltage and the operating temperature. The input capacitor maximum RMS input current
rating (I_RMS-IN) must be greater than:
I_RMS-IN = I_OUT x
Dx
1
-
D
+
r²
12
(10)
Neglecting inductor ripple simplifies the above equation to:
I_RMS-IN = I_OUT x _{D x 1 - D}
(11)
It can be shown from the above equation that maximum RMS capacitor current occurs when D = 0.5. Always
calculate the RMS at the point where the duty cycle D is closest to 0.5. The ESL of an input capacitor is usually
determined by the effective cross sectional area of the current path. As a rule of thumb, a large leaded capacitor
will have high ESL and a 1206 ceramic chip capacitor will have very low ESL. At the operating frequencies of the
LM2833, leaded capacitors may have an ESL so large that the resulting impedance (2πfL) will be higher than
that required to provide stable operation. It is strongly recommended to use ceramic capacitors due to their low
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