8
LTC34 48
3448f
OPERATIO
U
(Refer to Functional Diagram)
the V
OUT
pin and both the main and synchronous switches
are turned off. The control loop is stabilized by the load
capacitor and requires a minimum value of 2µF. The
LTC3448 will change back to switching mode and turn off
the LDO when the load current exceeds approximately
11mA.
When MODE is connected to an intermediate voltage level
(i.e., V
OUT
), this switchover is automatic. If MODE is pulled
high to V
IN
, the LDO remains on and the switcher off
regardless of the load current. The LDO is capable of
providing a maximum of approximately 15mA before the
load regulation will degrade to unacceptable levels. If
MODE is pulled to GND, the switcher remains on and the
LDO off regardless of the load current.
Figure 2. I
LDO(ON)
vs V
IN
, V
OUT
Figure 3. I
LDO(ON)
vs L
OUT
Some applications may be able to anticipate the transition
from high to low and low to high load currents. In these
cases it may be desirable to switch between modes by
controlling the MODE pin with a processor signal. In these
applications it is important that the MODE pin is pulled
high no earlier than 50µs after the RUN pin is pulled high.
This will ensure proper start-up of internal reference
circuitry.
The load current I
LDO(ON)
below which the switcher will
automatically turn off and the LDO turn on is independent
of the external capacitor, and to first order, independent
of supply and output voltage. There is an inverse relation-
ship between I
LDO(ON)
and the value of the inductor.
These dependencies are shown in Figures 2 and 3.
Automatic operation with inductor values below 1µH is
not recommended.
At the low load currents at which the switcher to linear
regulator transition occurs, the switcher is operating in
pulse skipping mode. During each switching cycle in this
mode, while the synchronous switch (bottom MOSFET) is
on, the inductor current decays until the reverse current
comparator is triggered. At this occurrence, the bottom
MOSFET is turned off. Ideally, this occurs when the
inductor current is precisely zero. In reality, because of on-
chip delays, this current will be negative at higher output
voltages.
The internal algorithm which controls the LDO turn-on
load current level makes certain assumptions about the
amount of charge transferred to the output on each
switching cycle. These assumptions are no longer met
when the inductor current begins to reverse. This causes
the load current at which the transition takes place to move
to lower levels at higher output voltages. For this reason
use of the LDO auto mode is not recommended for output
levels above 2V. For output voltages above 2V, the MODE
pin should be driven externally.
Short-Circuit Protection
When the output is shorted to ground, the main switch
cycle will be skipped, and the synchronous switch will
remain on for a longer duration. This allows the inductor
current more time to decay, thereby preventing runaway.
V
IN
(V)
2
I
LDO(ON)
(mA)
2.5
3.0
3.5
6
3448 F02
2.0
1.5
0
3
4
5
1.0
0.5
4.5
4.0
V
OUT
= 1.2V
V
OUT
= 1.5V
V
OUT
= 1.8V
T
A
= 25°C
L = 2.2µH
INDUCTOR VALUE (µH)
0
0
I
LDO(ON)
(mA)
1.0
2.0
3.0
2
4
68
3448 F03
10
4.0
5.0
0.5
1.5
2.5
3.5
4.5
12
V
IN
= 3.6V
V
OUT
= 1.5V
T
A
= 25°C
