LTC3780EG Linear Technology, LTC3780EG Datasheet - Page 22

• Manufacturer Part Number: LTC3780EG
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,CMOS,SSOP,24PIN,PLASTIC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3780
In Buck mode, maximum sense voltage and the sense
resistance determines the maximum allowed inductor
valley current, which is:
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 condi-
tions. 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 overvolt-
age condition is cleared. During an overvoltage condition,
a negative current limit (V
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
22
MOSFETs, sensing resistor, inductor and PC board
traces and cause the efficiency to drop at high output
currents.
I
L MAX BUCK
(
2
R losses. These arise from the resistances of the
,
)
=
130
R
U
SENSE
mV
U
SENSE
= –60mV) is set to limit
W
U
2. Transition loss. This loss arises from the brief amount
3. INTV
4. C
5. Other losses. Schottky diode D1 and D2 are responsible
When making adjustments to improve efficiency, the input
current is the best indicator of changes in efficiency. If you
make a change and the input current decreases, then the
efficiency has increased. If there is no change in input
current, then there is no change in efficiency.
Design Example
As a design example, assume V
nal), V
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:
where CR
and control currents. This loss can be reduced by
supplying INTV
a high efficiency source, such as an output derived
boost network or alternate supply if available.
job of filtering the large RMS input current to the regu-
lator in Buck mode. The output capacitor has the more
difficult job of filtering the large RMS output current in
Boost mode. Both C
ESR to minimize the AC I
tance to prevent the RMS current from causing addi-
tional upstream losses in fuses or batteries.
for conduction losses during dead time and light load
conduction periods. Inductor core loss occurs pre-
dominately at light loads. Switch C causes reverse
recovery current loss in Boost mode.
IN
Transition Loss ≈ 1.7A
and C
OUT
CC
current. This is the sum of the MOSFET driver
= 12V (5%), I
OUT
SS
is the reverse transfer capacitance.
loss. The input capacitor has the difficult
CC
current through the EXTV
IN
and C
OUT(MAX)
2
–1
R loss and sufficient capaci-
OUT
• V
IN
= 5V to 18V (12V nomi-
are required to have low
= 5A and f = 400kHz.
IN
2
• I
OUT
• C
CC
RSS
pin from
• f
3780f