Datasheet

LTC3812-5
13
38125fc
The four possible operating modes for generating this
supply are summarized as follows (see Figure 4):
1. LTC3812-5 generates a 5.5V start-up supply from a small
external SOT-23 NMOS acting as linear regulator with
drain connected to V
IN
and gate controlled by the
LTC3812-5’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 effi ciency. If the output is lost due to a short,
the LTC3812-5 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 SOT-23 NMOS. This scheme eliminates the long
start-up times associated with a conventional trickle
charger by using an external NMOS to quickly charge
the IC/driver supply capacitor (C
INTVCC
).
2. Similar to (1) except that the external NMOS is used
for continuous IC/driver power instead of just for start-
up. The NMOS is sized for proper dissipation and the
driver shutdown/restart for V
OUT
< 4.7V is disabled.
This scheme is less effi cient 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 NMOS. The IC/driver supply
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 4.2V 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 suffi cient 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 4.2V and 14V) is available
and connected directly to the IC/driver supply pins.
Figure 4. Operating Modes for IC/Driver Supply
OPERATION
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
4.2V TO
14V
38125 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
+
LTC3812-5 LTC3812-5
LTC3812-5 LTC3812-5
+
–