LTC3630_15 Datasheet, PDF (11/26 Pages) Linear Technology – High Efficiency, 65V 500mA Synchronous Step-Down Converter

LTC3630
Applications Information
The basic LTC3630 application circuit is shown on the front
page of the 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 guaranteed current limit
of 1A (1.2A typical), which guarantees a maximum average
load current of 500mA. For applications that demand less
current, the peak current threshold can be reduced to as
little as 100mA (120mA typical). This lower peak current
allows the use of lower value, smaller components (input
capacitor, output capacitor, and inductor), resulting in
lower supply ripple and a smaller overall DC/DC converter.
between efficiency and light load output voltage ripple, as
described in the C_ISET Selection section of the Applica-
tions Information. For maximum efficiency, minimize the
capacitance on the I_SET pin and place the R_ISET resistor
as close to the pin as possible.
The typical peak current is internally limited to be within the
range of 120mA to 1.2A. Shorting the I_SET pin to ground
programs the current limit to 120mA, and leaving it float
sets the current limit to the maximum value of 1.2A. When
selecting this resistor value, be aware that the 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 conditions. Selecting the
peak current to be 2.2 times greater than the maximum
load current is a good starting point for most applications.
The threshold can be easily programmed using a resis-
tor (R_ISET) between the I_SET pin and ground. The voltage
generated on the I_SET pin by R_ISET and the internal 5µA
current source sets the peak current. The voltage on the
I_SET pin is internally limited within the range of 0.1V to
1.0V. The value of resistor for a particular peak current can
be selected by using Figure 2 or the following equation:
R_ISET = I_PEAK • 0.2 • 10⁶
where 100mA < I_PEAK < 1A.
The internal 5μA current source is reduced to 1μA in sleep
mode to maximize efficiency and to facilitate a trade-off
220
200
180
160
140
120
100
80
60
40
20
0
50 100 150 200 250 300 350 400 450 500
MAXIMUM LOAD CURRENT (mA)
3630 F02
Figure 2. R_ISET Selection
Inductor Selection
The inductor, input voltage, output voltage, and peak cur-
rent determine the switching frequency during a burst
cycle of the LTC3630. For a given input voltage, output
voltage, and peak current, the inductor value sets the
switching frequency during a burst cycle when the output
is in regulation. Generally, switching between 50kHz and
250kHz yields high efficiency, and 200kHz is a good first
choice for many applications. The inductor value can be
determined by the following equation:
L
=



f
V_OUT
• I_PEAK



•

^1–
V_OUT
V_IN


The variation in switching frequency during a burst cycle
with input voltage and inductance is shown in Figure 3. For
lower values of I_PEAK, multiply the frequency in Figure 3
by 1.2A/I_PEAK.
An additional constraint on the inductor value is the
LTC3630’s 150ns minimum on-time of the high side switch.
Therefore, in order to keep the current in the inductor
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3630fd
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