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參數(shù)資料
| 型號： | LTC1871-7 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Quadruple 2-Input Positive-AND Gate 14-SOIC -40 to 85 |
| 中文描述： | 2相升壓型DC / DC控制器 |
| 文件頁數(shù)： | 11/36頁 |
| 文件大小： | 411K |
| 代理商： | LTC1871-7 |
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11
LTC1871
APPLICATIOU
W
U
U
As a result, high input voltage applications in which a large
power MOSFET is being driven at high frequencies can
cause the LTC1871 to exceed its maximum junction
temperature rating. The junction temperature 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)
The total quiescent current I
Q(TOT)
consists of the static
supply current (I
Q
) and the current required to charge and
discharge the gate of the power MOSFET. The 10-pin
MSOP package has a thermal resistance of R
TH(JA)
=
120
°
C/W.
As an example, consider a power supply with V
IN
= 5V and
V
O
= 12V at I
O
= 1A. The switching frequency is 500kHz,
and the maximum ambient temperature is 70
°
C. The
power MOSFET chosen is the IRF7805, which has a
maximum R
DS(ON)
of 11m
(at room temperature) and a
maximum total gate charge of 37nC (the temperature
coefficient of the gate charge is low).
I
Q(TOT)
= 600
μ
A + 37nC 500kHz = 19.1mA
P
IC
= 5V 19.1mA = 95mW
T
J
= 70
°
C + 120
°
C/W 95mW = 81.4
°
C
This demonstrates how significant the gate charge current
can be when compared to the static quiescent current in
the IC.
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked when
operating in a continuous mode at high V
IN
. A tradeoff
between the operating frequency and the size of the power
MOSFET may need to be made in order to maintain a
reliable IC junction temperature. Prior to lowering the
operating frequency, however, be sure to check with
power MOSFET manufacturers for their latest-and-great-
est low Q
G
, low R
DS(ON)
devices. Power MOSFET manu-
facturing technologies are continually improving, with
newer and better performance devices being introduced
almost yearly.
Output Voltage Programming
The output voltage is set by a resistor divider according to
the following formula:
V
V
R
R
O
=
+
1 230
.
1
2
1
The external resistor divider is connected to the output as
shown in Figure 1, allowing remote voltage sensing. The
resistors R1 and R2 are typically chosen so that the error
Figure 7. Bypassing the LDO Regulator and Gate Driver Supply
+
–
+
1.230V
R2
R1
P-CH
5.2V
DRIVER
GATE
C
VCC
4.7
μ
F
C
IN
INPUT
SUPPLY
2.5V TO 30V
GND
PLACE AS CLOSE AS
POSSIBLE TO DEVICE PINS
M1
1871 F07
INTV
CC
V
IN
GND
LOGIC
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