Datasheet

LTC3775
12
3775fa
APPLICATIONS INFORMATION
Output Overvoltage Protection
An overvoltage comparator, MAX, guards against transient
overshoots (>10%) as well as other more serious condi-
tions that may overvoltage the output. In such cases, the
top MOSFET is turned off and the bottom MOSFET is turned
on until the overvoltage condition is cleared.
Run/Shutdown
The LTC3775 can be put into a low power shutdown mode
with quiescent current <14µA by pulling the RUN/SHDN
pin below 0.74V. The RUN/SHDN pin can also be used as
an accurate external UVLO (undervoltage lockout) input
with a threshold of 1.22V. The driver outputs stay low if
this pin is <1.22V. The external resistive divider R4 and R5
shown in the Block Diagram can be used to set the UVLO
level based on V
IN
. The V
IN
voltage at which the switching
starts is given by the following formula:
UVLO (Upper) = 1.22V • (1 + R4/R5) – (1µA • R4)
The RUN/SHDN pin has an internal 1µA pull-up for default
turn-on if this pin is left fl oating. This 1µA pull-up current is
included in the above UVLO calculation. When RUN/SHDN
goes above 1.22V, this pull-up current is increased to 5µA.
This provides some amount of hysteresis to the UVLO
threshold. The lower UVLO level becomes:
UVLO (Lower) = 1.22V • (1 + R4/R5) – (5µA • R4)
So the amount of hysteresis is given by:
UVLO (Hysteresis) = 4µA • R4
Soft-Start
The LTC3775 includes a soft-start circuit that provides a
smooth output voltage ramp during start-up. The SS pin
requires an external capacitor, C
SS
, to GND with the value
determined by the required soft-start time. An internal 1µA
current source charges C
SS
. When the voltage on the SS
pin is less than the 0.6V internal reference, the LTC3775
regulates the V
FB
voltage to the SS pin voltage instead of
the 0.6V reference. As the SS voltage rises linearly from
0V to 0.6V and beyond, the output voltage, V
OUT
, rises
smoothly from zero to its fi nal value. The total soft-start
time can be calculated as:
t
C
μA
SOFTSTART
SS
=
09
1
.•
The SS pin is pulled low in the following conditions: during
an LDO undervoltage condition (INTV
CC
< 3.6V), during
shutdown (RUN pin < 1.22V), during an overtemperature
condition (T
J
> 165°C) and during current limit.
If either the top or bottom current limit comparator trips,
the SS pin is pulled low until the inductor current regu-
lates at around the current limit setting. Once the fault is
cleared, SS will start charging up allowing the duty cycle
and output voltage to increase gradually. Due to the cur-
rent limit action on the SS pin, it is important to avoid
an overcurrent condition during start-up of the power
supply, or V
OUT
will fail to start up properly.
0.74V
+
–
+
–
1.22V
TURN OFF TG
EN
4μA
RUN
R4
R5
3775 F02
I
RUN
1μA
V
IN
SHDN
LTC3775
CHIP
SHUTDOWN
Figure 2. RUN Pin Control
Figure 3. Typical Start-Up Waveform
for a Buck Converter Using the LTC3775
I
L
5A/DIV
V
OUT
0.5V/DIV
V
SS
1V/DIV
2ms/DIV
3775 F03
V
IN
= 12V
V
OUT
= 1.2V
C
SS
= 0.01μF
MODE = 0V
SW FREQ = 500kHz
SWITCHOVER
FROM PULSE-
SKIPPING TO
CONTINUOUS
MODE