LM5116MH National Semiconductor, LM5116MH Datasheet - Page 17

• Manufacturer Part Number: LM5116MH
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,TSSOP,20PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM5116MH

• Rohs Compliant: NO

For a 5V output, the maximum current sense signal occurs at
the minimum input voltage, so R
For this example VCCX = 0V, so V
sense resistor is calculated as:
The next lowest standard value of 10mΩ was chosen for R
RAMP CAPACITOR
With the inductor and sense resistor value selected, the value
of the ramp capacitor (C
ramp circuit is:
Where L is the value of the output inductor in Henrys, g
the ramp generator transconductance (5µA/V), and A is the
current sense amplifier gain (10V/V). For the 5V output design
example, the ramp capacitor is calculated as:
The next lowest standard value of 270pF was selected for
C
recommended.
OUTPUT CAPACITORS
The output capacitors smooth the inductor ripple current and
provide a source of charge for transient loading conditions.
For this design example, five 100µF ceramic capacitors
where selected. Ceramic capacitors provide very low equiv-
alent series resistance (ESR), but can exhibit a significant
reduction
manufacturer’s data, the ESR at 250kHz is 2mΩ / 5 =
0.4mΩ, with a 36% reduction in capacitance at 5V. This is
verified by measuring the output ripple voltage and frequency
response of the circuit. The fundamental component of the
output ripple voltage is calculated as:
With typical values for the 5V design example:
RAMP
. A COG type capacitor with 5% or better tolerance is
in
capacitance
RAMP
) necessary for the emulation
with
S
CS(TH)
is calculated from:
DC
= 0.11V. The current
bias.
From
m
the
S
is
.
17
INPUT CAPACITORS
The regulator supply voltage has a large source impedance
at the switching frequency. Good quality input capacitors are
necessary to limit the ripple voltage at the VIN pin while sup-
plying most of the switch current during the on-time. When the
buck switch turns on, the current into the switch steps to the
valley of the inductor current waveform, ramps up to the peak
value, and then drops to zero at turn-off. The input capacitors
should be selected for RMS current rating and minimum ripple
voltage. A good approximation for the required ripple current
rating is I
Quality ceramic capacitors with a low ESR were selected for
the input filter. To allow for capacitor tolerances and voltage
rating, four 2.2µF, 100V ceramic capacitors were used for the
typical application circuit. With ceramic capacitors, the input
ripple voltage will be triangular and peak at 50% duty cycle.
Taking into account the capacitance change with DC bias, the
input ripple voltage is approximated as:
When the converter is connected to an input power source, a
resonant circuit is formed by the line impedance and the input
capacitors. If step input voltage transients are expected near
the maximum rating of the LM5116, a careful evaluation of the
ringing and possible overshoot at the device VIN pin should
be completed. To minimize overshoot make C
The characteristic source impedance and resonant frequency
are:
The converter exhibits a negative input impedance which is
lowest at the minimum input voltage:
The damping factor for the input filter is given by:
When δ = 1, the input filter is critically damped. This may be
difficult to achieve with practical component values. With δ <
0.2, the input filter will exhibit significant ringing. If δ is zero or
negative, there is not enough resistance in the circuit and the
input filter will sustain an oscillation. When operating near the
minimum input voltage, an aluminum electrolytic capacitor
across C
bench test setup. Any parallel capacitor should be evaluated
for its RMS current rating. The current will split between the
ceramic and aluminum capacitors based on the relative
impedance at the switching frequency.
VCC CAPACITOR
The primary purpose of the VCC capacitor (C
the peak transient currents of the LO driver and bootstrap
diode (D1) as well as provide stability for the VCC regulator.
RMS
IN
may be needed to damp the input for a typical
> I
OUT
/ 2.
VCC
IN
) is to supply
www.national.com
> 10 x L
IN
.