LTC1474 LINER [Linear Technology], LTC1474 Datasheet - Page 12

• Manufacturer Part Number: LTC1474
• Description: Low Quiescent Current High Efficiency Step-Down Converters
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
LTC1474/LTC1475
with a built-in internal disconnect. The user needs to
assess his/her application to determine whether this situ-
ation could occur. If so, additional protection is necessary.
Prevention against latchup can be accomplished by sim-
ply connecting a Schottky diode across the SW and V
pins as shown in Figure 8. The diode will normally be
reverse biased unless V
time the diode will clamp the (V
than the 0.6V required for latchup. Note that a low leakage
Schottky should be used to minimize the effect on no-load
supply current. Schottky diodes such as MBR0530, BAS85
and BAT84 work well. Another more serious effect of the
protection diode leakage is that at no load with nothing to
provide a sink for this leakage current, the output voltage
can potentially float above the maximum allowable toler-
ance. To prevent this from occuring, a resistor must be
connected between V
enough to sink the maximum possible leakage current.
Thermal Considerations
In the majority of the applications, the LTC1474/LTC1475
do not dissipate much heat due to their high efficiency.
However, in applications where the switching regulator is
running at high ambient temperature with low supply
voltage and high duty cycles, such as dropout with the
switch on continuously, the user will need to do some
thermal analysis. The goal of the thermal analysis is to
determine whether the power dissipated by the regulator
exceeds the maximum junction temperature of the part.
The temperature rise is given by:
12
T
R
= P •
Figure 8. Preventing Absolute Maximum
Ratings from Being Exceeded
JA
V
IN
LTC1474
LTC1475
U
SW
OUT
IN
INFORMATION
U
is pulled below V
and ground with a value low
LATCHUP
PROTECTION
SCHOTTKY
OUT
– V
+
W
IN
1474/75 F08
) potential to less
V
OUT
OUT
U
at which
IN
where P is the power dissipated by the regulator and
is the thermal resistance from the junction of the die to the
ambient temperature.
The junction temperature is given by:
As an example consider the LTC1474/LTC1475 in dropout
at an input voltage of 3.5V, a load current of 300mA, and
an ambient temperature of 70 C. From the typical perfor-
mance graph of switch resistance, the on-resistance of the
P-channel switch at 70 C is 3.5 . Therefore, power dissi-
pated by the part is:
For the MSOP package, the
junction temperature of the regulator is:
which is near the maximum junction temperature of 125
Note that at higher supply voltages, the junction tempera-
ture is lower due to reduced switch resistance.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC1474/LTC1475. These items are also illustrated
graphically in the layout diagram of Figure 9. Check the
following in your layout:
1. Is the Schottky diode cathode closely connected to SW
2. Is the 0.1 F input decoupling capacitor closely con-
3. When using adjustable version, is the resistive divider
4. Is the 1000pF decoupling capacitor for the current
T
P = I
T
(Pin 5)?
nected between V
capacitor carries the high frequency peak currents.
closely connected to the (+) and (–) plates of C
a 10pF capacitor connected across R2?
sense resistor connected as close as possible to Pins 6
and 7? If no current sense resistor is used, Pins 6 and
7 should be shorted.
J
J
= T
= 70 C + (0.315)(150) = 117 C
2
A
• R
+ T
DS(ON)
R
= 0.315W
IN
(Pin 7) and ground (Pin 4)? This
JA
is 150 C/W. Thus the
OUT
with
o
JA
C.