LTC3780EG Linear Technology, LTC3780EG Datasheet - Page 19

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

APPLICATIO S I FOR ATIO
The R
the R
the case to the ambient temperature (R
of T
used in the iterative calculation process.
Schottky Diode (D1, D2) Selection
and Light Load Operation
The Schottky diodes D1 and D2 shown in Figure 1 conduct
during the dead time between the conduction of the power
MOSFET switches. They are intended to prevent the body
diode of Synchronous Switches B and D from turning on
and storing charge during the dead time. In particular, D2
significantly reduces reverse recovery current between
Switch D turn-off and Switch C turn-on, which improves
converter efficiency and reduces Switch C voltage stress.
In order for the diode to be effective, the inductance
between it and the synchronous switch must be as small
as possible, mandating that these components be placed
adjacently.
In Buck mode, when the FCB pin voltage is 0.85 < V
5V, the converter operates in Skip-Cycle mode. In this
mode, Synchronous Switch B remains off until the induc-
tor peak current exceeds one-fifth of its maximum peak
current. As a result, D1 should be rated for about one-half
to one-third of the full load current.
In Boost mode, when the FCB pin voltage is higher than
5.3V, the converter operates in Discontinuous Current
mode. In this mode, Synchronous Switch D remains off
until the inductor peak current exceeds one-fifth of its
maximum peak current. As a result, D2 should be rated for
about one-third to one-fourth of the full load current.
In Buck mode, when the FCB pin voltage is higher than
5.3V, the converter operates in constant frequency Dis-
continuous Current mode. In this mode, Synchronous
Switch B remains on until the inductor valley current is
lower than the sense voltage representing the minimum
negative inductor current level (V
Switch A and B are off until next clock signal.
In Boost mode, when the FCB pin voltage is 0.85 < V
5.3V, the converter operates in Burst Mode operation. In
this mode, the controller clamps the peak inductor current
J
TH(JC)
can then be compared to the original, assumed value
TH(JA)
for the device plus the thermal resistance from
to be used in the equation normally includes
U
U
W
SENSE
TH(JC)
= –5mV). Both
). This value
U
FCB
FCB
<
<
to approximately 20% of the maximum inductor current.
The output voltage ripple can increase during Burst Mode
operation.
INTV
An internal P-channel low dropout regulator produces 6V
at the INTV
the drivers and internal circuitry within the LTC3780. The
INTV
and must be bypassed to ground with a minimum of 4.7µF
tantalum, 10µF special polymer or low ESR type electro-
lytic capacitor. A 1µF ceramic capacitor placed directly
adjacent to the INTV
recommended. Good bypassing is necessary to supply
the high transient current required by MOSFET gate
drivers.
Higher input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the
maximum junction temperature rating for the LTC3780 to
be exceeded. The system supply current is normally
dominated by the gate charge current. Additional external
loading of the INTV
for the power dissipation calculations. The total INTV
current can be supplied by either the 6V internal linear
regulator or by the EXTV
applied to the EXTV
INTV
regulator. Power dissipation for the IC in this case is
V
tion temperature can be estimated by using the equations
given in Note 2 of the Electrical Characteristics. For
example, LTC3780 V
24mA from a 24V supply when not using the EXTV
as:
Use of the EXTV
ture to:
To prevent maximum junction temperature from being
exceeded, the input supply current must be checked
operating in continuous mode at maximum V
IN
T
T
• I
J
J
CC
CC
CC
= 70°C + 24mV • 24V • 95°C/W = 125°C
= 70°C + 24mV • 6V • 95°C/W = 84°C
INTVCC
Regulator
pin regulator can supply a peak current of 40mA
current is supplied by the internal 6V linear
CC
, and overall efficiency is lowered. The junc-
pin from the V
CC
input pin reduces the junction tempera-
CC
CC
also needs to be taken into account
IN
CC
pin is less than 5.7V, all of the
current is limited to less than
CC
and PGND IC pins is highly
IN
input pin. When the voltage
supply pin. INTV
LTC3780
IN
CC
.
19
powers
CC
3780f
pin
CC