LTC3780EG Linear Technology, LTC3780EG Datasheet - Page 21

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

APPLICATIO S I FOR ATIO
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
decoupled with a capacitor to ground, 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
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.
FCB Pin Regulates Secondary Winding in Buck Mode
In Buck mode, the FCB pin can be used to regulate a
secondary winding or as a logic level input. Continuous
operation is forced when the FCB pin drops below 0.8V.
During continuous mode, current flows continuously in
the transformer primary. The secondary winding(s) draw
current only when Switch B and Switch D are on in Buck
mode. When primary load currents are low and/or the
V
be on for a sufficient amount of time to transfer power
from the output capacitor to the secondary load. Forced
continuous operation will support secondary windings if
there is sufficient synchronous switch duty factor. Thus,
the FCB input pin removes the requirement that power
must be drawn from the auxiliary windings. With the loop
in continuous mode, the auxiliary outputs may nominally
be loaded without regard to the primary output load.
IN
/V
OUT
ratio is low, the Synchronous Switch B may not
CC
) will be on even when the controller is
U
U
W
R6
R5
FCB
SGND
LTC3780
Figure 10. Secondary Output Loop
U
BG2
TG2
V
SEC
V
IN
A
B
SW2
•
1:N
The secondary output voltage V
shown in Figure 10 by turns ratio N of the transformer:
However, if the controller goes into Burst Mode operation
and halts switching due to a light primary load current,
then V
to the FCB pin sets a minimum voltage V
If the V
temporary continuous switching operation until V
again above its minimum.
In order to prevent erratic operation if no external connec-
tions are made to FCB pin, the FCB pin has a 0.18µA
internal current source pulling the pin high. Include this
current when choosing resistor values R5 and R6.
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:
T1
R
3780 F10
V
I
•
SENSE
V
L MAX BOOST
SEC
SEC MIN
(
SW1
SEC
D
C
SEC
(
≈ (N + 1) • V
V
OUT
,
will drop. An external resistive divider from V
drops below this level, the FCB voltage forces
)
TG1
BG1
≈
0 8
)
. •
=
160
R
C
⎛
⎜
⎝
OUT
SENSE
OUT
1
+
mV
R
R
6
5
⎞
⎟
⎠
SEC
is normally set as
LTC3780
SEC(MIN)
:
21
SEC
SEC
3780f
is