LTC3631 Datasheet, PDF (10/22 Pages) Linear Technology – High Effi ciency, High Voltage 100mA Synchronous Step-Down Converter

LTC3631
APPLICATIONS INFORMATION
The basic LTC3631 application circuit is shown on the front
page of this data sheet. External component selection is
determined by the maximum load current requirement and
begins with the selection of the peak current programming
resistor, R_ISET. The inductor value L can then be determined,
followed by capacitors C_IN and C_OUT.
Peak Current Resistor Selection
The peak current comparator has a maximum current limit
of 225mA nominally, which results in a maximum aver-
age current of 112mA. For applications that demand less
current, the peak current threshold can be reduced to as
little as 50mA. This lower peak current allows the use of
lower value, smaller components (input capacitor, output
capacitor and inductor), resulting in lower input supply
ripple and a smaller overall DC/DC converter.
The threshold can be easily programmed with an ap-
propriately chosen resistor (R_ISET) between the I_SET pin
and ground. The value of resistor for a particular peak
current can be computed by using Figure 1 or the follow-
ing equation:
R_ISET = I_PEAK • 4.5 • 10⁶
where 50mA < I_PEAK < 225mA.
The peak current is internally limited to be within the
range of 50mA to 225mA. Shorting the I_SET pin to ground
programs the current limit to 50mA, and leaving it floating
sets the current limit to the maximum value of 225mA.
When selecting this resistor value, be aware that the
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MAXIMUM LOAD CURRENT (mA)
3631 F01
Figure 1. R_ISET Selection
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maximum average output current for this architecture
is limited to half of the peak current. Therefore, be sure
to select a value that sets the peak current with enough
margin to provide adequate load current under all foresee-
able operating conditions.
Inductor Selection
The inductor, input voltage, output voltage and peak cur-
rent determine the switching frequency of the LTC3631.
For a given input voltage, output voltage and peak current,
the inductor value sets the switching frequency when the
output is in regulation. A good first choice for the inductor
value can be determined by the following equation:


L=
V_OUT


•


1–
V_OUT


 ^{f • I}_PEAK   ^V_IN 
The variation in switching frequency with input voltage
and inductance is shown in the following two figures for
typical values of V_OUT. For lower values of I_PEAK, multiply
the frequency in Figure 2 and Figure 3 by 225mA/I_PEAK.
An additional constraint on the inductor value is the
LTC3631’s 100ns minimum on-time of the high side switch.
Therefore, in order to keep the current in the inductor well
controlled, the inductor value must be chosen so that it is
larger than L_MIN, which can be computed as follows:
L_MIN
=
V_IN(MAX) • t_ON(MIN)
I_PEAK(MAX)
where V_IN(MAX) is the maximum input supply voltage for
the application, t_ON(MIN) is 100ns, and I_PEAK(MAX) is the
maximum allowed peak inductor current. Although the
above equation provides the minimum inductor value,
higher efficiency is generally achieved with a larger inductor
value, which produces a lower switching frequency. For a
given inductor type, however, as inductance is increased
DC resistance (DCR) also increases. Higher DCR trans-
lates into higher copper losses and lower current rating,
both of which place an upper limit on the inductance. The
recommended range of inductor values for small surface
mount inductors as a function of peak current is shown in
Figure 4. The values in this range are a good compromise
between the tradeoffs discussed above. For applications
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