LM3489EVAL National Semiconductor, LM3489EVAL Datasheet - Page 11

• Manufacturer Part Number: LM3489EVAL
• Description: BOARD EVALUATION LM3489
• Manufacturer: National Semiconductor

Specifications of LM3489EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 3.3V
• Current - Output: 500mA
• Voltage - Input: 7 ~ 28V
• Regulator Topology: Buck
• Board Type: Fully Populated
• Utilized Ic / Part: LM3489
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -
• Frequency - Switching: -

where V
V
Design Information
Hysteretic control is a simple control scheme. However the
operating frequency and other performance characteristics
highly depend on external conditions and components. If ei-
ther the inductance, output capacitance, ESR, V
changed, there will be a change in the operating frequency
and output ripple. The best approach is to determine what
operating frequency is desirable in the application and then
begin with the selection of the inductor and C
INDUCTOR SELECTION (L)
The important parameters for the inductor are the inductance
and the current rating. The LM3489 operates over a wide fre-
quency range and can use a wide range of inductance values.
A rule of thumb is to use the equations used for National's
Simple Switchers
a function of output current (I
Δi
Δi
The inductance can be calculated based upon the desired
operating frequency where:
And
where D is the duty cycle, V
V
EN_HYST
DS
for I
for I
is the voltgae drop across the PFET.
≤
≤
out
out
I
I
out
out
< 2.0Amps
> 2.0Amps
EN
is the enable threshold hysteresis.
x I
x 0.3
out
is the enable rising threshold voltage and
−0.366726
®
. The equation for inductor ripple (Δi) as
D
is the diode forward voltage, and
OUT
) is:
OUT
IN
ESR.
, or Cff is
11
The inductor should be rated to the following:
The inductance value and the resulting ripple is one of the key
parameters controlling operating frequency. The second is
the inductor ESR that contribute to the steady state power
loss due to current flowing through the inductor.
OUTPUT CAPACITOR SELECTION (C
The ESR of the output capacitor times the inductor ripple cur-
rent is equal to the output ripple of the regulator. However, the
V
creased with a given inductance, operating frequency in-
creases as well. If ESR is reduced then the operating
frequency reduces.
The use of ceramic capacitors has become a common desire
of many power supply designers. However, ceramic capaci-
tors have a very low ESR resulting in a 90° phase shift of the
output voltage ripple. This results in low operating frequency
and increased output ripple. To fix this problem a low value
resistor should be added in series with the ceramic output
capacitor. Although counter intuitive, this combination of a
ceramic capacitor and external series resistance provides
highly accurate control over the output voltage ripple. Other
types capacitor, such as Sanyo POS CAP and OS-CON,
Panasonic SP CAP, and Nichicon "NA" series, are also rec-
ommended and may be used without additional series resis-
tance.
For all practical purposes, any type of output capacitor may
be used with proper circuit verification.
INPUT CAPACITOR SELECTION (C
A bypass capacitor is required between the input source and
ground. It must be located near the source pin of the external
PFET. The input capacitor prevents large voltage transients
at the input and provides the instantaneous current when the
PFET turns on.
The important parameters for the input capacitor are the volt-
age rating and the RMS current rating. Follow the
manufacturer's recommended voltage derating. For high in-
put voltage applications, low ESR electrolytic, Nichicon "UD"
series or the Panasonic "FK" series are available. The RMS
current in the input capacitor can be calculated as follows:
HYST
sets the first order value of this ripple. As ESR is in-
FIGURE 6. Adjustable UVLO
IN
)
OUT
20186930
)
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