LTC3531-3 Linear Technology, LTC3531-3 Datasheet - Page 10

• Manufacturer Part Number: LTC3531-3
• Description: Buck-Boost Synchronous DC/DC Converters
• Manufacturer: Linear Technology

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OPERATIO
LTC3531/
LTC3531-3.3/LTC3531-3
Buck Mode
If V
buck or step-down mode. The higher offset between V
and V
netizing voltage across the inductor when the R
are taken into account. At the beginning of a buck mode
cycle (Figure 1 right side) switches A and D are turned
on (V
is delivered to the output and ramped up until I
detected. When this occurs, A is turned off, B is turned
on and inductor current falls (–V
tor) until an I
sequence is repeated until the output is pumped above
its regulation voltage, a fi nal I
part returns to sleep mode.
Start-Up Mode
Before V
disabled and its body diode is used to transfer current to
the output capacitor. In start-up mode, the I
sense circuit is disabled and an alternate algorithm is used
to control inductor current. When the LTC3531 is brought
out of shutdown (assuming V
A and C are turned on until the inductor current reaches
I
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
AC switch-on to I
~800ns is repeated until V
10
PEAK
IN
. The AC switches are then turned off and inductor
IN
OUT
is ~800mV above V
– V
OUT
(800mV) is required to ensure suffi cient mag-
OUT
reaches approximately 1.6V, the D switch is
VALLEY
is applied across the inductor), current
PEAK
U
OFF
is detected. This AD then BD switch
timer to ~800nS. This sequence of
then B switch and D body diode for
OUT
OUT
OUT
, the LTC3531 operates in
ZERO
reaches ~1.6V. Once this
is discharged) switches
OUT
is detected, and the
across the induc-
VALLEY
PEAK
DSONS
/I
ZERO
IN
is
threshold is reached, the LTC3531 will transfer through the
required modes until V
Due to propagation delays in the sense circuitry, the
magnitudes of the I
shift depending on V
OTHER LTC3531 FEATURES
Shutdown: The part is shut down by pulling ⎯ S ⎯ H ⎯ D ⎯ N below
0.4V, and made active by pulling the pin up to V
Note that ⎯ S ⎯ H ⎯ D ⎯ N can be driven above V
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
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.
PEAK
IN
, V
OUT
, I
OUT
VALLEY
is brought into regulation.
and operating mode.
, and I
ZERO
IN
or V
currents may
OUT
IN
OUT
, as long
or V
to go
OUT
3531f
.