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參數(shù)資料
| 型號： | LTC3806EDE |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Synchronous Flyback DC/DC Controller |
| 中文描述： | 0.05 A SWITCHING CONTROLLER, 290 kHz SWITCHING FREQ-MAX, PDSO12 |
| 封裝： | 4 X 3 MM, PLASTIC, MO-229WGED, DFN-12 |
| 文件頁數(shù)： | 9/20頁 |
| 文件大小： | 241K |
| 代理商： | LTC3806EDE |
9
LTC3806
3806f
APPLICATIOU
INTV
CC
Regulator Bypassing and Operation
An internal voltage regulator produces the 6.9V supply
that powers the gate drivers and logic circuitry within the
LTC3806. The INTV
CC
regulator can supply up to 50mA
and must be bypassed to ground immediately adjacent to
the IC pins with a minimum of 4.7
μ
F ceramic capacitor.
Good bypassing is necessary to supply the high transient
currents required by the MOSFET gate drivers.
In an actual application, most of the IC supply current is
used to drive the gate capacitances of the power MOSFETs.
As a result, high input voltage applications with large
power MOSFETs can cause the LTC3806 to exceed its
maximum junction temperature rating. The junction tem-
perature can be estimated using the following equations:
I
Q(TOT)
= I
Q
+ f Q
G
P
IC
= V
IN
(I
Q
+ f
Q
G
)
T
J
= T
A
+ P
IC
R
TH(JA)
where
I
Q
is the static supply current
Q
G
is the total gate charge of all external power MOSFETs
P
IC
is the power dissipated in the IC
f is the switching frequency, nominally 250kHz
R
TH(JA)
is the package thermal resistance, junction to
ambient, nominally 34
°
C/W for the 12-pin DFN package
As an example, consider a 2-output power supply that
uses an Si7450DP primary-side power MOSFET, that has
a maximum total gate charge of 42nC and two Si4840DY
power MOSFETs (one for each output), each of which has
28nC maximum total gate charge.
The total gate charge is:
Q
G
=
42nC + 2
28nC = 98nC
The total supply current is:
I
Q(TOT)
= 2000
μ
A + 98nC
250kHz = 27mA
This demonstrates how significant the gate charge current
can be when compared to static quiescent current in the
IC.
W
U
U
If V
IN
is set to 10V, the power dissipation is:
P
IC
= 10
27mA = 270mW
and the junction temperature (assuming 70 degree ambi-
ent temperature) is:
T
J
= 70
°
C + 270mW
120
°
C/W = 102.4
°
C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked when
operating at high V
IN
. If junction temperature is too high,
using a separate transformer winding to lower V
IN
may be
tried. Prior to adding an additional transformer winding
(which raises transformer cost), be sure to check with
power MOSFET manufacturers for their newest low Q
G
,
low R
DS(ON)
devices. Power MOSFET manufacturing tech-
nologies are continually improving, with newer and better
performance devices being introduced almost yearly.
Output Voltage Programming
This IC will generally be used in DC/DC converters with
multiple outputs. The output voltage of the master output
(V
OUT1
) is set by a resistor divider according to the
following formula:
V
V
R
R
OUT1
1 230
.
1
6
7
=
+
The external resistor divider is connected as shown in
Figure 1. The resistors R6 and R7 are typically chosen so
that the error caused by the current flowing into the FB pin
during normal operation is less than 1% (this translates to
a maximum value of R7 of about 120k).
The nominal slave output (V
OUT2
) voltage is set according
to the following formula:
V
OUT2
= V
OUT1
N21
where N21 is the turns ratio of the transformer windings
between V
OUT2
and V
OUT1
.
If additional slave outputs are added their voltage is
determined by the equation:
V
OUTN
= V
OUT1
N
N1
where N
N1
is the turns ratio of the transformer windings
between V
OUTN
and V
OUT1
.
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