LTC3812-5_15 Datasheet, PDF (13/34 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3812-5
OPERATION
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 efficiency. 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 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 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 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 4.2V and 14V) is available
and connected directly to the IC/driver supply pins.
Mode 1: MOSFET for Start-Up Only
V_IN
I > 270μA
NDRV
INTV_CC
LTC3812-5
+
5.5V
EXTV_CC
V_OUT (> 4.7V)
Mode 3: Trickle Charge Mode ^VIN
Mode 2: MOSFET for Continuous Use
V_IN
I < 270μA
NDRV
INTV_CC
LTC3812-5
+
5.5V
EXTV_CC
Mode 4: External Supply
NDRV
INTV_CC
LTC3812-5
+ ^5.5V
NDRV
INTV_CC
LTC3812-5
EXTV_CC
V_OUT
EXTV_CC
Figure 4. Operating Modes for IC/Driver Supply
+
+–
4.2V TO
14V
38125 F04
38125fc
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