ltc3612 Linear Technology Corporation, ltc3612 Datasheet - Page 13

• Manufacturer Part Number: ltc3612
• Description: 3a, 4mhz Monolithic Synchronous Step-down Dc/dc Converter
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
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 LTC3612, the input voltage and the inductor value.
This foldback behavior helps in limiting the peak inductor
The basic LTC3612 application circuit is shown in Figure 1.
Operating Frequency
Selection of the operating frequency is a trade-off between
effi ciency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves effi ciency 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 LTC3612 is determined
by an external resistor that is connected between the
RT/SYNC pin and ground. The value of the resistor sets
2.25V TO 5.5V
V
IN
R
2M
SS
C
22nF
C
470pF
SS
C
15k
R
C
Figure 1. 1.8V, 3A Step-Down Regulator
C
10pF
(OPT)
C1
R
130k
T
RUN
TRACK/SS
RT/SYNC
PGOOD
ITH
MODE
SV
IN
LTC3612
PV
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 6A typical, the top
power MOSFET will be held off and switching cycles will
be skipped until the inductor current is reduced.
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:
Although frequencies as high as 4MHz are possible, the
minimum on-time of the LTC3612 imposes a minimum
limit on the operating duty cycle. The minimum on-time
is typically 60ns; therefore, the minimum duty cycle is
equal to 100 • 60ns • f
Tying the RT/SYNC pin to SV
operating frequency to 2.25MHz ±20%.
PV
V
IN_DRV
SGND
PGND
FB
IN
R
DDR
R2
196k
SW
T
=
3.82 • 10
f
470nH
OSC
392k
R1
L1
C
22μF
( )
IN1
Hz
11
3612 F01
Hz
OSC
C
47μF
Ω – 16k
C
22μF
OUT1
IN2
(Hz)%.
IN
C
22μF
OUT2
sets the default internal
V
1.8V
3A
OUT
LTC3612
13
3612f