LM2854MH-500/NOPB National Semiconductor, LM2854MH-500/NOPB Datasheet - Page 13

• 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

LM2854. The input capacitor should be placed as close as
possible to the PVIN and PGND pins to substantially eliminate
the parasitic effects of any stray inductance or resistance on
the PC board and supply lines. Additional bulk capacitance
with higher ESR may be required to damp any resonance ef-
fects of the input capacitance and parasitic inductance.
AVIN FILTERING COMPONENTS
In addition to the large input filter capacitor, a smaller ceramic
capacitor such as a 0.1 µF or 1.0 µF is recommended between
AVIN and AGND to filter high frequency noise present on the
PVIN rail from the quiet AVIN supply. For additional filtering
in noisy environments, a small RC filter can be used on the
AVIN pin as shown below.
In general, R
that the steady state voltage drop across the resistor due to
the AVIN bias current does not affect the UVLO level. Rec-
ommended filter capacitor, C
dielectric.
SOFT-START CAPACITOR
When the LM2854 is enabled, the output voltage will ramp up
linearly in the time dictated by the following relationship
where V
I
C
equation allows for the necessary soft-start capacitor for a
given startup time to be calculated as follows
Thus, the required soft start capacitor per unit output voltage
startup time is given by
For example, a 10 nF soft-start capacitor will yield a 4 ms soft-
start time.
TRACKING - EQUAL SOFT-START TIME
One way to use the tracking feature is to design the tracking
resistor divider so that the master supply output voltage,
V
gether and reach their target values at the same time. This is
termed ratiometric startup. For this case, the equation gov-
erning the values of tracking divider resistors R
given by
SS
OUT1
SS
is the soft-start charging current (nominally 2 µA) and
is the external soft-start capacitance. Rearranging this
, and the LM2854 output voltage, V
REF
is the internal reference voltage (nominally 0.8V),
F
is typically selected between 1Ω and 10Ω so
C
SS
= 2.5 nF / ms
F
, is 1.0 µF in X5R or X7R
OUT2
, both rise to-
T1
and R
30052840
T2
is
13
The above equation includes an offset voltage to ensure that
the final value of the SS pin voltage exceeds the reference
voltage of the LM2854. This offset will cause the LM2854 out-
put voltage to reach regulation slightly before the master
supply. A value of 33 kΩ 1% is recommended for R
compromise between high precision and low quiescent cur-
rent through the divider while minimizing the effect of the 2 µA
soft-start current source.
For example, If the master supply voltage V
the LM2854 output voltage was 1.8V, then the value of R
needed to give the two supplies identical soft-start times
would be 14.3 kΩ. A timing diagram for this example, the
equal soft-start time case, is shown below.
TRACKING - EQUAL SLEW RATES
Alternatively, the tracking feature can be used to have similar
output voltage ramp rates. This is referred to as simultaneous
startup. In this case, the tracking resistors can be determined
based on the following equation
and to ensure proper overdrive of the SS pin
For the example case of V
RT2 set to 33 kΩ as before, R
equation to be 15.5 kΩ. A timing diagram for the case of equal
slew rates is shown below.
V
OUT2
OUT1
< 0.8 V
T1
= 5V and V
is calculated from the above
OUT1
OUT2
OUT1
= 2.5V, with
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is 3.3V and
30052859
T2
as a
T1