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參數(shù)資料
| 型號(hào)： | LTC1778-1 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Wide Operating Range, No RSENSE Step-Down Controller |
| 中文描述： | 寬工作，無檢測(cè)電阻范圍步降控制器 |
| 文件頁數(shù)： | 14/24頁 |
| 文件大小： | 303K |
| 代理商： | LTC1778-1 |
14
LTC1778/LTC1778-1
1778fa
but an additional ceramic capacitor in parallel is recom-
mended to reduce the effect of their lead inductance.
Top MOSFET Driver Supply (C
B
, D
B
)
An external bootstrap capacitor C
B
connected to the BOOST
pin supplies the gate drive voltage for the topside MOSFET.
This capacitor is charged through diode D
B
from INTV
CC
when the switch node is low. When the top MOSFET turns
on, the switch node rises to V
IN
and the BOOST pin rises
to approximately V
IN
+ INTV
CC
. The boost capacitor needs
to store about 100 times the gate charge required by the
top MOSFET. In most applications 0.1
μ
F to 0.47
μ
F, 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 0.8V 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
IN
.
Tying the FCB pin below the
0.8V threshold forces continuous synchronous operation,
allowing current to reverse at light loads and maintaining
high frequency operation.
In addition to providing a logic input to force continuous
operation, the FCB pin provides a means to maintain a
flyback winding output when the primary is operating in
discontinuous mode. The secondary output V
OUT2
is nor-
mally set as shown in Figure 6 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
OUT2
will droop. An external resistor
divider from V
OUT2
to the FCB pin sets a minimum voltage
V
OUT2(MIN)
below which continuous operation is forced
until V
OUT2
has risen above its minimum.
V
V
R
R
OUT MIN
2
0 8
.
1
4
3
(
)
=
+
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 LTC1778, the maximum sense voltage is controlled by
the voltage on the V
RNG
pin. With valley current control,
the maximum sense voltage and the sense resistance
determine the maximum allowed inductor valley current.
The corresponding output current limit is:
I
V
R
I
LIMIT
SNS MAX
(
DS ON
(
T
L
=
+
)
)
ρ
1
2
The current limit value should be checked to ensure that
I
LIMIT(MIN)
> I
OUT(MAX)
. The minimum value of current limit
generally occurs with the largest V
IN
at the highest ambi-
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
LIMIT
which heats
the MOSFET switches.
Caution should be used when setting the current limit
based upon the R
DS(ON)
of the MOSFETs. The maximum
current limit is determined by the minimum MOSFET on-
resistance. Data sheets typically specify nominal and
maximum values for R
DS(ON)
, but not a minimum. A
reasonable assumption is that the minimum R
DS(ON)
lies
the same amount below the typical value as the maximum
lies above it. Consult the MOSFET manufacturer for further
guidelines.
To further limit current in the event of a short circuit to
ground, the LTC1778 includes foldback current limiting. If
the output falls by more than 25%, then the maximum
sense voltage is progressively lowered to about one sixth
of its full value.
APPLICATIOU
W
U
U
Figure 6. Secondary Output Loop and EXTV
CC
Connection
V
IN
LTC1778
EXTV
CC
SGND
FCB
TG
SW
OPTIONAL
EXTV
CC
CONNECTION
5V < V
OUT2
< 7V
R3
R4
1778 F06
T1
1:N
BG
PGND
+
C
OUT2
1
μ
V
OUT1
V
OUT2
V
IN
+
C
IN
1N4148
+
C
OUT
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