LM3753 Datasheet, PDF (20/38 Pages) National Semiconductor (TI) – Scalable 2-Phase Synchronous Buck Controllers with Integrated FET Drivers and Linear Regulator Controller

In order to track properly, the external power supply voltage
must be higher than the LM3753 output voltage.
SOFT-START (LM3754)
To avoid current limit during startup, the soft-start time t_SS
should be substantially longer than the time required to
charge C_OUT to V_OUT at the maximum output current. To meet
this requirement:
Choose a soft-start capacitor according to the formula:
Where C_SS is the soft-start capacitor and t_SS is the soft-start
time.
External Components Selection
The following is a design example selecting components for
the Typical Application Schematic of Figure 20. The circuit is
designed for two controller 4-phase operation with 1.2V out
at 100A from an input voltage of 6V to 18V. The expected load
is a microprocessor or ASIC with fast load transients, and the
type of MOSFETs used are in SO-8 or its equivalent packages
such as PowerPAK ®, PQFN and LFPAK (LFPAK-i).
SWITCHING FREQUENCY
The selection of switching frequency is based on the tradeoff
between size, cost, and efficiency. In general, a lower fre-
quency means larger, more expensive inductors and capac-
itors will be needed. A higher switching frequency generally
results in a smaller but less efficient solution. For this appli-
cation a frequency of 300 kHz was selected as a good com-
promise between the size of the inductor and MOSFETs,
transient response and efficiency. Following the equation giv-
en for R_FRQ in the Functional Description under OSCILLATOR
and SYNCHRONIZATION, for 300 kHz operation a 78.7 kΩ
1% resistor is used for R_FRQ. A 1000 pF capacitor is used for
C_FRQ.
OUTPUT INDUCTORS
The first criterion for selecting an output inductor is the induc-
tance itself. In most buck converters, this value is based on
the desired peak-to-peak ripple current, ΔI_L that flows in the
inductor along with the load current. As with switching fre-
quency, the selection of the inductor is a tradeoff between size
and cost. Higher inductance means lower ripple current and
hence lower output voltage ripple. Lower inductance results
in smaller, less expensive devices. An inductance that gives
a ripple current of 1/5 to 2/5 of the maximum output current is
a good starting point. (ΔI_L = (1/5 to 2/5) x I_OUT). Minimum in-
ductance is calculated from this value, using the maximum
input voltage as:
The second criterion is inductor saturation current rating. The
LM3753/54 has an accurately programmed peak current limit.
During an output short circuit, the inductor should be chosen
so as not to exceed its saturation rating at elevated temper-
ature. For the design example, a standard value of 440 nH is
chosen to fall within the ΔI_L = (1/5 to 2/5) x I_OUT range.
The dc loss in the inductor is determined by its series resis-
tance R_L. The dc power dissipation is found from:
P_DC = I_OUT2 x R_L
The ac loss can be estimated from the inductor
manufacturer’s data, if available. The ac loss is set by the
peak-to-peak ripple current ΔI_L and the switching frequency
f_SW.
OUTPUT CAPACITORS
The output capacitors filter the inductor ripple current and
provide a source of charge for transient load conditions. A
wide range of output capacitors may be used with the
LM3753/54 that provides excellent performance. The best
performance is typically obtained using aluminum electrolytic,
tantalum, polymer, solid aluminum, organic or niobium type
chemistries in parallel with ceramic capacitors. The ceramic
capacitors provide extremely low impedance to reduce the
output ripple voltage and noise spikes, while the aluminum or
other capacitors provide a larger bulk capacitance for tran-
sient loading.
When selecting the value for the output capacitors the two
performance characteristics to consider are the output volt-
age ripple and transient response. The output voltage ripple
for a single phase can be approximated as:
With all values normalized to a single phase, ΔV_O (V) is the
peak to peak output voltage ripple, ΔI_L (A) is the peak to peak
inductor ripple current, R_C (Ω) is the equivalent series resis-
tance or ESR of the output capacitors, f_SW (Hz) is the switch-
ing frequency, and C_O (F) is the output capacitance. The
amount of output ripple that can be tolerated is application
specific. A general recommendation is to keep the output rip-
ple less than 1% of the rated output voltage. Figure 11 shows
the output voltage ripple for multi-phase operation.
By calculating in terms of amperes, volts, and megahertz, the
inductance value will come out in micro henries. The inductor
ripple current is found from the minimum inductance equation:
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