LTC1625 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1625' PDF : 24 Pages View PDF
LTC1625
APPLICATIONS INFORMATION
Note that RDS(ON) also varies with the gate drive level. If
gate drives other than the 5.2V INTVCC are used, this must
be accounted for when selecting the MOSFET RDS(ON).
Particular care should be taken with applications where
EXTVCC is connected to the output. When the output
voltage is between 4.7V and 5.2V, INTVCC will be con-
nected to the output and the gate drive is reduced. The
resulting increase in RDS(ON) will also lower the current
limit. Even applications with VOUT > 5.2V will traverse this
region during start-up and must take into account the
reduced current limit.
Topside MOSFET Driver Supply (CB, DB)
An external bootstrap capacitor (CB in the functional
diagram) connected to the BOOST pin supplies the gate
drive voltage for the topside MOSFET. This capacitor is
charged through diode DB from INTVCC when the SW node
is low. Note that the voltage across CB is about a diode
drop below INTVCC. When the top MOSFET turns on, the
switch node voltage rises to VIN and the BOOST pin rises
to approximately VIN + INTVCC. During dropout operation,
CB 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 VPROG pin as follows:
VPROG = 0V
VPROG = INTVCC
VPROG = Open
VOUT = 3.3V
VOUT = 5V
VOUT = Adjustable
Remote sensing of the output voltage is provided by the
VOSENSE pin. For fixed 3.3V and 5V output applications an
internal resistive divider is used and the VOSENSE pin is
connected directly to the output voltage as shown in
Figure 6a. When using an external resistive divider, the
VPROG pin is left open and the VOSENSE pin is connected to
feedback resistors as shown in Figure 6b. The output
voltage is set by the divider as:
VOUT
=
1.19V1+
R2
R1
VOUT = 5V: INTVCC
VOUT = 3.3V: GND
LTC1625
VPROG
VOUT
+
VOSENSE
COUT
SGND
1625 F06a
Figure 6a. Fixed 3.3V or 5V VOUT
OPEN
+
R2
COUT
R1
LTC1625
VPROG
VOSENSE
SGND
1625 F06b
Figure 6b. Adjustable VOUT
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 VIN by gradually in-
creasing the controller’s current limit ITH(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 (IQ < 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 CSS. If RUN/SS has
been pulled all the way to ground there is a delay before
starting of approximately:
14
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