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LTC3780_12 Datasheet, PDF (21/28 Pages) Linear Technology – High Effi ciency, Synchronous, 4-Switch Buck-Boost Controller | |||
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LTC3780
APPLICATIONS INFORMATION
The Standby Mode (STBYMD) Pin Function
The standby mode (STBYMD) pin provides several choices
for start-up and standby operational modes. If the pin is
pulled to ground, the SS pin is internally pulled to ground,
preventing start-up and thereby providing a single control
pin for turning off the controller. If the pin is left open or
bypassed to ground with a capacitor, the SS pin is internally
provided with a starting current, permitting external control
for turning on the controller. If the pin is connected to a
voltage greater than 1.25V, the internal regulator (INTVCC)
will be on even when the controller is shut down (RUN
pin voltage < 1.5V). In this mode, the onboard 6V linear
regulator can provide power to keep-alive functions such
as a keyboard controller.
Fault Conditions: Overvoltage Protection
A comparator monitors the output for overvoltage con-
ditions. The comparator (OV) detects overvoltage faults
greater than 7.5% above the nominal output voltage.
When the condition is sensed, switches A and C are
turned off, and switches B and D are turned on until the
overvoltage condition is cleared. During an overvoltage
condition, a negative current limit (VSENSE = â60mV) is
set to limit negative inductor current. When the sensed
current inductor current is lower than â60mV, switch A and
C are turned on, and switch B and D are turned off until
the sensed current is higher than â20mV. If the output is
still in overvoltage condition, switch A and C are turned
off, and switch B and D are turned on again.
Fault Conditions: Current Limit and Current Foldback
The maximum inductor current is inherently limited in a
current mode controller by the maximum sense voltage.
In boost mode, maximum sense voltage and the sense
resistance determines the maximum allowed inductor
peak current, which is:
IL(M AX ,BOOST )
=
160mV
RSENSE
In buck mode, maximum sense voltage and the sense
resistance determines the maximum allowed inductor
valley current, which is:
IL(MAX,BUCK )
=
130mV
RSENSE
To further limit current in the event of a short circuit to
ground, the LTC3780 includes foldback current limiting.
If the output falls by more than 30%, then the maximum
sense voltage is progressively lowered to about one third
of its full value.
Efï¬ciency Considerations
The percent efï¬ciency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efï¬ciency and which change would
produce the most improvement. Although all dissipative
elements in circuit produce losses, four main sources
account for most of the losses in LTC3780 circuits:
1. DC I2R losses. These arise from the resistances of the
MOSFETs, sensing resistor, inductor and PC board
traces and cause the efï¬ciency to drop at high output
currents.
2. Transition loss. This loss arises from the brief amount
of time switch A or switch C spends in the saturated
region during switch node transitions. It depends upon
the input voltage, load current, driver strength and
MOSFET capacitance, among other factors. The loss
is signiï¬cant at input voltages above 20V and can be
estimated from:
Transition Loss â 1.7Aâ1 ⢠VIN2 ⢠IOUT ⢠CRSS ⢠f
where CRSS is the reverse transfer capacitance.
3780fe
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