Datasheet

LTC3780
22
3780ff
For more information www.linear.com/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 (INTV
CC
)
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: 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:
I
L(MAX,BOOST)
=
160mV
R
SENSE
In buck mode, maximum sense voltage and the sense
resistance determines the maximum allowed inductor
valley current, which is:
I
L(MAX,BUCK)
=
130mV
R
SENSE
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.
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 (V
SENSE
= –60mV) is set to limit
negative inductor current. When the sensed current in
-
ductor current is lower than –60mV, switch A and C are
turned on, and switch B and D are turned off until the
sensed current is higher than –20mV. If the output is still
in overvoltage condition, switch A and C are turned off,
and switch B and D are turned on again.
Efficiency Considerations
The percent efficiency 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 efficiency 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 I
2
R losses. These arise from the resistances of the
MOSFETs, sensing resistor, inductor and PC board
traces and cause the efficiency 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 significant at input voltages above 20V and can be
estimated from:
Transition Loss ≈ 1.7A
–1
• V
IN2
• I
OUT
• C
RSS
• f
where C
RSS
is the reverse transfer capacitance.