Datasheet

14
LTC1625
Note that R
DS(ON)
also varies with the gate drive level. If
gate drives other than the 5.2V INTV
CC
are used, this must
be accounted for when selecting the MOSFET R
DS(ON)
.
Particular care should be taken with applications where
EXTV
CC
is connected to the output. When the output
voltage is between 4.7V and 5.2V, INTV
CC
will be con-
nected to the output and the gate drive is reduced. The
resulting increase in R
DS(ON)
will also lower the current
limit. Even applications with V
OUT
> 5.2V will traverse this
region during start-up and must take into account the
reduced current limit.
Topside MOSFET Driver Supply (C
B
, D
B
)
An external bootstrap capacitor (C
B
in the functional
diagram) connected to the BOOST pin supplies the gate
drive voltage for the topside MOSFET. This capacitor is
charged through diode D
B
from INTV
CC
when the SW node
is low. Note that the voltage across C
B
is about a diode
drop below INTV
CC
. When the top MOSFET turns on, the
switch node voltage rises to V
IN
and the BOOST pin rises
to approximately V
IN
+ INTV
CC
. During dropout operation,
C
B
supplies the top driver for as long as ten cycles between
refreshes. Thus, the boost capacitance needs to store
about 100 times the gate charge required by the top
MOSFET. In many applications 0.22µF is adequate.
When adjusting the gate drive level , the final arbiter is the
total input current for the regulator. If you make a change
and the input current decreases, then you improved the
efficiency. If there is no change in input current, then there
is no change in efficiency.
Output Voltage Programming
The LTC1625 has a pin selectable output voltage deter-
mined by the V
PROG
pin as follows:
V
PROG
= 0V V
OUT
= 3.3V
V
PROG
= INTV
CC
V
OUT
= 5V
V
PROG
= Open V
OUT
= Adjustable
Remote sensing of the output voltage is provided by the
V
OSENSE
pin. For fixed 3.3V and 5V output applications an
internal resistive divider is used and the V
OSENSE
pin is
connected directly to the output voltage as shown in
Figure 6a. When using an external resistive divider, the
APPLICATIONS INFORMATION
WUU
U
V
PROG
pin is left open and the V
OSENSE
pin is connected to
feedback resistors as shown in Figure 6b. The output
voltage is set by the divider as:
VV
R
R
OUT
=+






119 1
2
1
.
V
PROG
V
OUT
= 5V: INTV
CC

V
OUT
= 3.3V: GND
LTC1625
V
OSENSE
1625 F06a
SGND
C
OUT
V
OUT
+
Figure 6a. Fixed 3.3V or 5V V
OUT
V
PROG
OPEN
LTC1625
V
OSENSE
1625 F06b
SGND
C
OUT
R1
R2
+
Figure 6b. Adjustable V
OUT
Run/Soft Start Function
The RUN/SS pin is a dual purpose pin that provides a soft
start function and a means to shut down the LTC1625. Soft
start reduces surge currents from V
IN
by gradually in-
creasing the controller’s current limit I
TH(MAX)
. This pin
can also be used for power supply sequencing.
Pulling the RUN/SS pin below 1.4V puts the LTC1625 into
a low quiescent current shutdown (I
Q
< 30µA). This pin can
be driven directly from logic as shown in Figure 7. Releas-
ing the RUN/SS pin allows an internal 3µA current source
to charge up the external capacitor C
SS
. If RUN/SS has
been pulled all the way to ground there is a delay before
starting of approximately: