LTC3810-5
14
38105fd
<4.7V or an external supply in the appropriate range can
be used. The LTC3810-5 will automatically detect which
mode is being used and operate properly.
The four possible operating modes for generating this
supply are summarized as follows (see Figure 4):
1. LTC3810-5 generates a 5.5V start-up supply from a small
external SOT23 N-channel MOSFET acting as linear
regulator with drain connected to V
IN
and gate controlled
by the LTC3810-5’s internal linear regulator controller
through the NDRV pin. As soon as the output voltage
reaches 4.7V, the 5.5V IC/driver supply is derived from
the output through an internal low-dropout regulator to
optimize efficiency. If the output is lost due to a short,
the LTC3810-5 goes through repeated low duty cycle
soft-start cycles (with the drivers shut off in between)
to attempt to bring up the output without burning up
the SOT23 MOSFET. This scheme eliminates the long
start-up times associated with a conventional trickle
charger by using an external MOSFET to quickly charge
the IC/driver supply capacitors (C
INTVCC
, C
DRVCC
).
2. Similar to (1) except that the external MOSFET is used
for continuous IC/driver power instead of just for
start-up. The MOSFET is sized for proper dissipation and
the driver shutdown/restart for V
OUT
< 4.7V is disabled.
This scheme is less efficient but may be necessary if
V
OUT
< 4.7V and a boost network is not desired.
3. Trickle charge mode provides an even simpler approach
by eliminating the external MOSFET. The IC/driver sup-
ply capacitors are charged through a single high-valued
resistor connected to the input supply. When the INTV
CC
voltage reaches the turn-on threshold of 9V (automati-
cally raised from 4.7V to provide extra headroom for
start-up), the drivers turn on and begin charging up the
output capacitor. When the output reaches 4.7V, IC/driver
power is derived from the output. In trickle-charge mode,
the supply capacitors must have sufficient capacitance
such that they are not discharged below the 4V INTV
CC
UV threshold before the output is high enough to take
over or else the power supply will not start.
4. Low voltage supply available. The simplest approach is
if a low voltage supply (between 4.5V and 14V) is avail-
able and connected directly to the IC/driver supply pins.
Figure 4. Operating Modes for IC/Driver Supply
NDRV
EXTV
CC
INTV
CC
V
OUT
(> 4.7V)
V
IN
I < 270µA
V
OUT
+
–
Mode 1: MOSFET for Start-Up Only
Mode 2: MOSFET for Continuous Use
Mode 3: Trickle Charge Mode
Mode 4: External Supply
5.5V
4.5V to
14V
38105 F04
NDRV
EXTV
CC
INTV
CC
NDRV
EXTV
CC
INTV
CC
NDRV
EXTV
CC
INTV
CC
V
IN
I > 270µA
5.5V
+
+
+
V
IN
5.5V
+
LTC3810-5
LTC3810-5
LTC3810-5
LTC3810-5
operaTion
