ltc3770 Linear Technology Corporation, ltc3770 Datasheet - Page 15

• Manufacturer Part Number: ltc3770
• Description: Fast No Rsense Step-down Synchronous Controller With Margining, Tracking And Pll
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Top MOSFET Driver Supply (C
An external bootstrap capacitor C
pin supplies the gate drive voltage for the topside MOSFET.
This capacitor is charged through diode D
when the switch node is low. When the top MOSFET turns
on, the switch node rises to V
to approximately V
to store about 100 times the gate charge required by the
top MOSFET. In most applications 0.1µF to 0.47µF, X5R or
X7R dielectric capacitor is adequate.
Discontinuous Mode Operation and FCB Pin
The FCB pin determines whether the bottom MOSFET
remains on when current reverses in the inductor. Tying
this pin above its 0.6V threshold enables discontinuous
operation where the bottom MOSFET turns off when
inductor current reverses. The load current at which
current reverses and discontinuous operation begins de-
pends on the amplitude of the inductor ripple current and
will vary with changes in V
0.6V threshold forces continuous synchronous operation,
allowing current to reverse at light loads and maintaining
high frequency operation. To prevent forcing current back
into the main power supply, potentially boosting the input
supply to a dangerous voltage level, forced continuous
mode of operation is disabled when the TRACK/SS voltage
is 20% below the reference voltage during soft-start or
tracking up. Forced continuous mode of operation is also
disabled when the TRACK/SS voltage is below 0.1V during
tracking down operation. During these two periods, the
PGOOD signal is forced low.
R4
R3
FCB
SGND
LTC3770
Figure 5. Secondary Output Loop
PGND
IN
U
SW
V
TG
BG
IN
+ INTV
U
IN
CC
. Tying the FCB pin below the
+
IN
. The boost capacitor needs
•
B
B
1:N
, D
and the BOOST pin rises
T1
C
V
1N4148
connected to the BOOST
IN
IN
B
W
•
)
+
+
C
B
OUT
C
1µF
from INTV
OUT2
3770 F05
V
V
U
OUT2
OUT1
CC
In addition to providing a logic input to force continuous
operation, the FCB pin provides a mean to maintain a
flyback winding output when the primary is operating in
discontinuous mode. The secondary output V
mally set as shown in Figure 5 by the turns ratio N of the
transformer. However, if the controller goes into discon-
tinuous mode and halts switching due to a light primary
load current, then V
divider from V
V
until V
Fault Conditions: Current Limit and Foldback
The maximum inductor current is inherently limited in a
current mode controller by the maximum sense voltage. In
the LTC3770, the maximum sense voltage is controlled by
the voltage on the V
the maximum sense voltage and the sense resistance
determine the maximum allowed inductor valley current.
The corresponding output current limit is:
The current limit value should be checked to ensure that
I
generally occurs with the largest V
ent temperature, conditions that cause the largest power
loss in the converter. Note that it is important to check for
self-consistency between the assumed MOSFET junction
temperature and the resulting value of I
the MOSFET switches.
Caution should be used when setting the current limit
based upon the R
current limit is determined by the minimum MOSFET on-
resistance. Data sheets typically specify nominal and
maximum values for R
reasonable assumption is that the minimum R
the same percentage below the typical value as the maxi-
mum lies above it. Consult the MOSFET manufacturer for
further guidelines.
LIMIT(MIN)
OUT2(MIN)
I
V
LIMIT
OUT MIN
OUT2
2
(
=
> I
below which continuous operation is forced
has risen above its minimum.
R
V
OUT(MAX)
)
DS ON
SNS MAX
OUT2
=
(
0 6
(
DS(ON)
.
to the FCB pin sets a minimum voltage
)
OUT2
V
RNG
ρ
. The minimum value of current limit
⎛
⎜
⎝
)
T
1
+ ∆
+
DS(ON)
pin. With valley current control,
of the MOSFETs. The maximum
will droop. An external resistor
R
R
1
2
3
4
⎞
⎟
⎠
I
L
, but not a minimum. A
IN
at the highest ambi-
LIMIT
LTC3770
which heats
OUT2
DS(ON)
15
is nor-
3770fb
lies