Datasheet

LTC1871-1
11
18711fb
APPLICATIONS INFORMATION
Figure 7. Bypassing the LDO Regulator and Gate Driver Supply
+
–
+
1.230V
R2 R1
P-CH
5.2V
DRIVER
GATE
C
VCC
4.7µF
C
IN
INPUT
SUPPLY
2.5V TO 30V
GND
PLACE AS CLOSE AS
POSSIBLE TO DEVICE PINS
M1
18711 F07
INTV
CC
V
IN
GND
LOGIC
As a result, high input voltage applications in which a
large power MOSFET is being driven at high frequencies
can cause the LTC1871-1 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
= 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 coeffi cient of
the gate charge is low).
I
Q(TOT)
= 600µA + 37nC • 500kHz = 19.1mA
P
IC
= 5V • 19.1mA = 95mW
T
J
= 70°C + 120°C/W • 95mW = 81.4°C
This demonstrates how signifi cant 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 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
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.
The resistors R1 and R2 are typically chosen so that the