LTC3810-5 Datasheet, PDF (14/36 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3810-5
OPERATION
<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 available
and connected directly to the IC/driver supply pins.
Mode 1: MOSFET for Start-Up Only
V_IN
I > 270μA
Mode 2: MOSFET for Continuous Use
V_IN
I < 270μA
NDRV
INTV_CC
LTC3810-5
+
5.5V
NDRV
INTV_CC
LTC3810-5
+
5.5V
EXTV_CC
V_OUT (> 4.7V)
Mode 3: Trickle Charge Mode ^VIN
EXTV_CC
Mode 4: External Supply
14
NDRV
INTV_CC
LTC3810-5
+
5.5V
NDRV
INTV_CC
LTC3810-5
EXTV_CC
V_OUT
EXTV_CC
Figure 4. Operating Modes for IC/Driver Supply
+
+ _{4.5V to}
– 14V
38105 F04
38105fc