LTM4624
10
4624fc
For more information www.linear.com/LTM4624
applicaTions inForMaTion
Discontinuous Current Mode (DCM)
In applications where low output ripple and high efficiency
at intermediate current are desired, discontinuous current
mode (DCM) should be used by connecting the MODE pin
to SGND. At light loads the internal current comparator
may remain tripped for several cycles and force the top
MOSFET to stay off for several cycles, thus skipping cycles.
The inductor current does not reverse in this mode.
Forced Continuous Current Mode (CCM)
In applications where fixed frequency operation is more
critical than low current efficiency, and where the lowest
output ripple is desired, forced continuous operation
should be used. Forced continuous operation can be
enabled by tying the MODE pin to INTV
CC
. In this mode,
inductor current is allowed to reverse during low output
loads, the COMP voltage is in control of the current
comparator threshold throughout, and the top MOSFET
always turns on with each oscillator pulse. During start-up,
forced continuous mode is disabled and inductor current
is prevented from reversing until the LTM4624’s output
voltage is in regulation.
Operating Frequency
The operating frequency of the LTM4624 is optimized to
achieve the compact package size and the minimum out-
put ripple voltage while still keeping high efficiency. The
default operating frequency is internally set to 1MHz. In
most applications, no additional frequency adjusting is
required.
If any operating frequency other than 1MHz is required
by application, the operating frequency can be increased
by adding a resistor, R
FSET
, between the FREQ pin and
SGND, as shown in Figure 21. The operating frequency
can be calculated as:
f Hz
( )
=
162k||R
FSET
Ω
( )
The operating frequency can also be decreased by adding
a resistor between the FREQ pin and INTV
CC
, calculated as:
f Hz
( )
= 1MHz –
R
FSET
Ω
( )
The programmable operating frequency range is from
800kHz to 4MHz.
Soft-Start And Output Voltage Tracking
The TRACK/SS pin provides a means to either soft start
the regulator or track it to a different power supply. A ca
-
pacitor on the TRACK/SS pin will program the ramp rate
of the output voltage. An internal 2.5µA current sour
ce
will charge up the external soft-start capacitor towards
INT
V
CC
voltage. When the TRACK/SS voltage is below
0.6V, it will take over the internal 0.6V reference voltage
to control the output voltage. The total soft-start time can
be calculated as:
t
SS
= 0.6•
C
SS
2.5µA
where C
SS
is the capacitance on the TRACK/SS pin. Cur-
rent foldback and forced continuous mode are disabled
during the soft-start process.
