LTC3531 Datasheet, PDF (10/16 Pages) Linear Technology – 200mA Buck-Boost Synchronous DC/DC Converters

LTC3531/
LTC3531-3.3/LTC3531-3
OPERATIO
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 sufficient 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 final 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 flows 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
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 rectifiers. 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.
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