Datasheet

LTC3531/
LTC3531-3.3/LTC3531-3
10
3531fa
Buck Mode
If V
IN
is ~800mV above V
OUT
, the LTC3531 operates in
buck or step-down mode. The higher offset between V
IN
and V
OUT
(800mV) is required to ensure suffi cient mag-
netizing voltage across the inductor when the R
DSONS
are taken into account. At the beginning of a buck mode
cycle (Figure 1 right side) switches A and D are turned
on (V
IN
– V
OUT
is applied across the inductor), current
is delivered to the output and ramped up until I
PEAK
is
detected. When this occurs, A is turned off, B is turned
on and inductor current falls (–V
OUT
across the induc-
tor) until an I
VALLEY
is detected. This AD then BD switch
sequence is repeated until the output is pumped above
its regulation voltage, a fi nal I
ZERO
is detected, and the
part returns to sleep mode.
Start-Up Mode
Before V
OUT
reaches approximately 1.6V, the D switch is
disabled and its body diode is used to transfer current to
the output capacitor. In start-up mode, the I
VALLEY
/I
ZERO
sense circuit is disabled and an alternate algorithm is used
to control inductor current. When the LTC3531 is brought
out of shutdown (assuming V
OUT
is discharged) switches
A and C are turned on until the inductor current reaches
I
PEAK
. The AC switches are then turned off and inductor
current fl ows to the output through the B switch and D
body diode. The period for the B switch/D body diode is
controlled by the t
OFF
timer to ~800nS. This sequence of
AC switch-on to I
PEAK
then B switch and D body diode for
~800ns is repeated until V
OUT
reaches ~1.6V. Once this
OPERA TIO
U
threshold is reached, the LTC3531 will transfer through the
required modes until V
OUT
is brought into regulation.
Due to propagation delays in the sense circuitry, the
magnitudes of the I
PEAK
, I
VALLEY
, and I
ZERO
currents may
shift depending on V
IN
, V
OUT
and operating mode.
OTHER LTC3531 FEATURES
Shutdown: The part is shut down by pulling
SHDN below
0.4V, and made active by pulling the pin up to V
IN
or V
OUT
.
Note that
SHDN an be driven above V
IN
or V
OUT
, as long
as it is limited to less than 6V.
Output Disconnect and Inrush Limiting: The LTC3531
is designed to allow true output disconnect by opening
both P-channel MOSFET rectifi ers. This allows V
OUT
to go
to zero volts during shutdown, drawing no current from
the input source. It also provides inrush current limiting
at turn-on, minimizing surge currents seen by the input
supply.
Thermal Shutdown: If the die temperature reaches ap-
proximately 150°C, the part will go into thermal shut-
down and all switches will be turned off. The part will be
enabled again when the die temperature has dropped by
10°C (nominal). To deliver the power that the LTC3531
is capable of, it is imperative that a good thermal path be
provided to dissipate the heat generated within the pack-
age. It is recommended that multiple vias in the printed
circuit board be used to conduct heat away from the IC
and into a copper plane with as much area as possible.
Soldering the Exposed Pad to the GND plane (DFN version)
is recommended to improve thermal performance.