LTC1871 Linear Technology, LTC1871 Datasheet - Page 11

• Manufacturer Part Number: LTC1871
• Description: Wide Input Range/ No RSENSE Current Mode Boost/ Flyback and SEPIC Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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:
The total quiescent current I
supply current (I
discharge the gate of the power MOSFET. The 10-pin
MSOP package has a thermal resistance of R
120 C/W.
As an example, consider a power supply with V
V
and the maximum ambient temperature is 70 C. The
power MOSFET chosen is the IRF7805, which has a
maximum R
maximum total gate charge of 37nC (the temperature
coefficient of the gate charge is low).
O
I
P
T
I
P
T
= 12V at I
Q(TOT)
Q(TOT)
J
J
IC
IC
= T
= 70 C + 120 C/W • 95mW = 81.4 C
= V
= 5V • 19.1mA = 95mW
A
IN
= 600 A + 37nC • 500kHz = 19.1mA
+ P
I
• (I
DS(ON)
Q
O
IC
+ f • Q
= 1A. The switching frequency is 500kHz,
Q
• R
Q
+ f • Q
) and the current required to charge and
of 11m (at room temperature) and a
TH(JA)
U
G
G
)
1.230V
U
R2
Q(TOT)
Figure 7. Bypassing the LDO Regulator and Gate Driver Supply
–
+
W
consists of the static
LOGIC
R1
IN
U
= 5V and
DRIVER
TH(JA)
P-CH
5.2V
=
INTV
GATE
GND
V
CC
IN
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
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
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:
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
V
1871 F07
O
+
1 230
C
4.7 F
G
VCC
.
PLACE AS CLOSE AS
POSSIBLE TO DEVICE PINS
, low R
V
C
•
IN
DS(ON)
1
M1
R
R
2
1
devices. Power MOSFET manu-
INPUT
SUPPLY
2.5V TO 30V
GND
LTC1871
IN
. A tradeoff
11