LTC3780EG Linear Technology, LTC3780EG Datasheet - Page 13

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

OPERATIO
The duty cycle of Switch C decreases until the minimum
duty cycle of the converter in Buck mode reaches
D
where D
switch range:
and f is the operating frequency in Hz.
Figure 5 shows typical boost mode waveforms. If V
approaches V
SWITCH D
LOW CURRENT OPERATION
The FCB pin is a multifunction pin providing two functions:
1) to provide regulation for a secondary winding by
temporarily forcing continuous PWM operation in Buck
mode and 2) to select among three modes for both buck
and boost operations by accepting a logic input. Figure 6
shows the different modes.
SWITCH A
SWITCH B
SWITCH C
MIN_BOOST
0V to 0.75V
0.85V to 5V
CLOCK
D
D
FCB PIN
MIN_BOOST
BUCK-BOOST
>5.3V
I
BUCK-BOOST
, given by:
Figure 6. Different Operating Modes
Figure 5. Boost Mode (V
OUT
DCM with Constant Freq
= (D
Force Continuous Mode
= (200ns • f) • 100%
, the Buck-Boost region is reached.
U
Skip-Cycle Mode
BUCK MODE
BUCK-BOOST
is the duty cycle of the Buck-Boost
) • 100%
IN
DCM with Constant Freq
< V
Force Continuous Mode
Burst Mode Operation
OUT
BOOST MODE
)
3780 F05
2.4V
IN
0V
When the FCB pin voltage is lower than 0.8V, the control-
ler behaves as a continuous, PWM current mode syn-
chronous switching regulator. In Boost mode, Switch A is
always on. Switch C and Synchronous Switch D are
alternately turned on to maintain the output voltage
independent of direction of inductor current. Every ten
cycles, Switch A is forced off for about 300ns to allow C
to recharge. In Buck mode, Synchronous Switch D is
always on. Switch A and Synchronous Switch B are
alternately turned on to maintain the output voltage inde-
pendent of direction of inductor current. Every ten cycles,
Synchronous Switch D is forced off for about 300ns to
allow C
mode at light load, but may be desirable in certain appli-
cations. In this mode, the output can source or sink
current. The sunk current will be forced back into the main
power supply potentially boosting the input supply to
dangerous voltage levels—BEWARE!
When the FCB pin voltage is below V
greater than 0.8V, the controller enters Burst Mode opera-
tion in Boost operation or enters Skip-Cycle mode in Buck
operation. During Boost operation, Burst Mode operation
sets a minimum output current level before inhibiting the
switch C and turns off Synchronous Switch D when the
inductor current goes negative. This combination of re-
quirements will, at low currents, force the I
voltage threshold that will temporarily inhibit turn-on of
power switches C and D until the output voltage drops.
There is 100mV of hysteresis in the burst comparator tied
to the I
the MOSFETs C and D that turn them on for several cycles,
followed by a variable “sleep” interval depending upon the
load current. The maximum output voltage ripple is limited
to 3% of the nominal DC output voltage as determined by
a resistive feedback divider. During buck operation, Skip-
Cycle mode sets a minimum positive inductor current
level. When inductor current is lower than this level,
Synchronous Switch B is kept off. In every cycle, the body
TH
B
to recharge. This is the least efficient operating
pin. This hysteresis produces output signals to
INTVCC
LTC3780
TH
pin below a
– 1V, but
13
3780f
A