LM2854MH-500/NOPB National Semiconductor, LM2854MH-500/NOPB Datasheet - Page 15

• Manufacturer Part Number: LM2854MH-500/NOPB
• Description: IC REG 4A SYNC SMPL SW 16ETSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®, SIMPLE SWITCHER®r
• Type: Step-Down (Buck)r

Specifications of LM2854MH-500/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 5 V
• Current - Output: 4A
• Frequency - Switching: 500kHz
• Voltage - Input: 2.95 ~ 5.5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: *LM2854MH-500; *LM2854MH-500/NOPB; LM2854MH-500

where the constant α is nominally 0.038 or 0.075 for the 500
kHz or 1 MHz options, respectively. This assumes a com-
pensator pole cancels the output capacitor ESR zero. Fur-
thermore, since the modulator gain is proportional to V
loop crossover frequency increases with V
ommended to design the loop at maximum expected V
The upper feedback resistor, R
equate mid-band gain and to locate a zero at or below the LC
pole frequency. The series resistor, R
cate a pole at the ESR zero frequency. Thus
Note that the lower feedback resistor, R
the control loop from an AC standpoint since the FB pin is the
input to an error amplifier and effectively at AC ground.
Hence, the control loop can be designed irrespective of output
voltage level. The only caveat here is the necessary derating
of the output capacitance with applied voltage. Having chosen
R
voltage.
Table 1 and Table 2 list inductor and ranges of capacitor val-
ues that work well with the LM2854, along with the associated
compensation components to ensure stable operation. Val-
ues different than those listed may be used, but the compen-
sation components may need to be recalculated to avoid
degradation in phase margin. Note that the capacitance
ranges specified refer to in-circuit values where the nominal
capacitance value is adequately derated for applied voltage.
FILTER INDUCTOR AND OUTPUT CAPACITOR
SELECTION
In a buck regulator, selection of the filter inductor and capac-
itor will affect many key system parameters, including stabil-
ity, transient response and efficiency The LM2854 can
accommodate relatively wide ranges of output capacitor and
filter inductor values in a typical application and still achieve
excellent load current transient performance and low output
voltage ripple.
The inductance is chosen such that the peak-to-peak inductor
current ripple, Δi
lows
FB1
as above, R
L
, is approximately 25 to 40% of I
FB2
is then selected for the desired output
FB1
, is selected to provide ad-
COMP
FB2
, has no impact on
, is selected to lo-
IN
. Thus, it is rec-
OUT
as fol-
IN
IN
, the
.
15
Note that the peak inductor current is the DC output current
plus half the ripple current and reaches its highest level at
lowest duty cycle (or highest V
inductor should have a saturation current rating in excess of
the current limit level.
When operating the LM2854 at input voltages above 5.2V, the
inductor should be sized to keep the minimum inductor cur-
rent above -0.5A. For most applications this should only occur
at light loads or when the inductor is drastically undersized.
To ensure the current never goes below -0.5A for any appli-
cation, the peak-to-peak ripple current (Δi
should be less than 1A. Keeping the minimum inductor cur-
rent above -0.5A limits the energy storage in the inductor and
helps prevent the switch node voltage from exceeding the
absolute maximum specification when the low side FET turns
off.
Table 3 lists examples of off-the-shelf powdered iron and fer-
rite based inductors that are suitable for use with the LM2854.
The output capacitor can be of ceramic or electrolytic chem-
istry. The chosen output capacitor requires sufficient DC volt-
age rating and RMS ripple current handling capability.
The output capacitor RMS current and peak-to-peak output
ripple are given respectively by
In general, 22 µF to 100 µF of ceramic output capacitance is
sufficient for both LM2854 frequency options given the opti-
mal high frequency characteristics and low ESR of ceramic
dielectric. It is advisable to consult the manufacturer’s derat-
ing curves for capacitance voltage coefficient as the in-circuit
capacitance may drop significantly with applied voltage.
Tantalum or organic polymer electrolytic capacitance may be
suitable with the LM2854 500 kHz option, particularly in ap-
plications where substantial bulk capacitance per unit volume
is required. However, the high loop bandwidth achievable with
the LM2854 obviates the necessity for large bulk capacitance
during transient loading conditions.
Table 4 lists some examples of commercially available ca-
pacitors that can be used with the LM2854.
IN
). It is recommended that the
L
) in the inductor
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