LTC1871-7_15 Datasheet, PDF (11/34 Pages) Linear Technology – High Input Voltage, Current Mode Boost, Flyback and SEPIC Controller

Operation
LTC1871-7
1.230V
R2
–
+
LOGIC
V_IN
INPUT
SUPPLY
6V TO 30V
P-CH
C_IN
R1
7V INTVCC
DRIVER
GATE
C_VCC
4.7µF
X5R
6V-RATED
M1
POWER
MOSFET
GND
18717 F07
GND
PLACE AS CLOSE AS
POSSIBLE TO DEVICE PINS
Figure 7. Bypassing the LDO Regulator and Gate Driver Supply
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.
18717fd
11