LTC3614_15 Datasheet, PDF (13/30 Pages) Linear Technology – 4A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter

LTC3614
Operation
If the output voltage drops below 50% of its nominal value,
the clamp voltage at ITH pin is lowered causing the maxi-
mum peak inductor current to decrease gradually with the
output voltage. When the output voltage reaches 0V the
clamp voltage at the ITH pin drops to 40% of the clamp
voltage during normal operation. The short-circuit peak
inductor current is determined by the minimum on-time
of the LTC3614, the input voltage and the inductor value.
This foldback behavior helps in limiting the peak inductor
current when the output is shorted to ground. It is disabled
during internal or external soft-start and tracking up/down
operation (see the Applications Information section).
A secondary limit is also imposed on the valley inductor
current. If the inductor current measured through the
bottom MOSFET increases beyond 12A typical, the top
power MOSFET will be held off and switching cycles will
be skipped until the inductor current is reduced.
Applications Information
The basic LTC3614 application circuit is shown in Figure 1.
Operating Frequency
Selection of the operating frequency is a trade-off between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves efficiency by
reducing internal gate charge losses but requires larger
inductance values and/or capacitance to maintain low
output ripple voltage.
The operating frequency of the LTC3614 is determined
by an external resistor that is connected between the RT/
SYNC pin and ground. The value of the resistor sets the
ramp current that is used to charge and discharge an
internal timing capacitor within the oscillator and can be
calculated by using the following equation:
R_T
=
3.82 • 10¹¹Hz
f_{OSC (}Hz₎
Ω
–
16kΩ
Although frequencies as high as 4MHz are possible, the
minimum on-time of the LTC3614 imposes a minimum
limit on the operating duty cycle. The minimum on-time
is typically 60ns; therefore, the minimum duty cycle is
equal to 60ns • f_OSC(Hz)•100%.
Tying the RT/SYNC pin to SV_IN sets the default internal
operating frequency to 2.25MHz ±20%.
V_IN
2.25V TO 5.5V
R_SS
2M
C_SS
22nF
R_C
15k
C_C
470pF
R_T
130k
C_C1
10pF
(OPT)
SV_IN
PV_IN
RUN
TRACK/SS SRLIM/DDR
RT/SYNC
LTC3614 SW
PGOOD
SGND
ITH
PGND
MODE
V_FB
R2
196k
C_IN1
10µF
×4
L1
330nH
R1
392k
C_OUT2
100µF
V_OUT
1.8V
4A
3614 F01
Figure 1. 1.8V, 4A Step-Down Regulator
For more information www.linear.com/LTC3614
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