Datasheet

LTC1871-7
11
18717fd
operaTion
can cause the LTC1871-7 to exceed its maximum junc-
tion 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
=10V.
The switching frequency is 200kHz, and the maximum
ambient temperature is 70°C. The power MOSFET chosen
is the FDS3670(Fairchild), which has a maximum R
DS(ON)
of 35mΩ (at room temperature) and a maximum total
gate charge of 80nC (the temperature coefficient of the
gate charge is low).
I
Q(TOT)
= 600µA + 80nC • 200kHz = 16.6mA
P
IC
= 10V • 16.6mA = 166mW
T
J
= 70°C + 120°C/W • 166mW = 89.9°C
T
JRISE
= 19.9°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-greatest low
QG, low R
DS(ON)
devices. Power MOSFET manufacturing
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
O
= 1.230V • 1+
R2
R1






The external resistor divider is connected to the output
as shown in Figure 1, allowing remote voltage sensing.
Figure 7. Bypassing the LDO Regulator and Gate Driver Supply
–
+
1.230V
R2
R1
P-CH
7V
DRIVER
GATE
C
VCC
4.7µF
X5R
C
IN
INPUT
SUPPLY
6V TO 30V
GND
PLACE AS CLOSE AS
POSSIBLE TO DEVICE PINS
M1
18717 F07
INTV
CC
V
IN
GND
LOGIC
6V-RATED
POWER
MOSFET