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LTC1475IS8#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
10
LTC1474/LTC1475
APPLICATIONS INFORMATION
WUU
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small that this loss is negligible at loads above a
milliamp but at no load accounts for nearly all of the
loss. The second component, the gate charge current,
results from switching the gate capacitance of the
internal P-channel switch. Each time the gate is switched
from high to low to high again, a packet of charge dQ
moves from V
IN
to ground. The resulting dQ/dt is the
current out of V
IN
which is typically much larger than the
DC bias current. In continuous mode, I
GATECHG
= fQ
P
where Q
P
is the gate charge of the internal switch. Both
the DC bias and gate charge losses are proportional to
V
IN
and thus their effects will be more pronounced at
higher supply voltages.
2. I
2
R losses are predicted from the internal switch,
inductor and current sense resistor. At low supply
voltages where the switch on-resistance is higher and
the switch is on for longer periods due to higher duty
cycle, the switch losses will dominate. Keeping the peak
currents low with the appropriate R
SENSE
and with
larger inductance helps minimize these switch losses.
At higher supply voltages, these losses are proportional
to load and result in the flat efficiency curves seen in
Figure 1.
3. The catch diode loss is due to the V
D
I
D
loss as the diode
conducts current during the off-time and is more pro-
nounced at high supply voltage where the on-time is
short. This loss is proportional to the forward drop.
However, as discussed in the Catch Diode section,
diodes with lower forward drops often have higher
leakage current, so although efficiency is improved, the
no load supply current will increase.
Adjustable Applications
For adjustable versions, the output voltage is programmed
with an external divider from V
OUT
to V
FB
(Pin 1) as shown
in Figure 4. The regulated voltage is determined by:
V
OUT
=1.23 1+
R2
R1
(4)
To minimize no-load supply current, resistor values in the
megohm range should be used. The increase in supply
current due to the feedback resistors can be calculated
from:
∆=
+
I
V
RR
V
V
VIN
OUT OUT
IN
12
A 10pF feedforward capacitor across R2 is necessary due
to the high impedances to prevent stray pickup and
improve stability.
GND
V
FB
LTC1474
LTC1475
1
4
1474/75 F04
R2
R1
10pF
V
OUT
LBI
LBO
LTC1474/LTC1475
1474/75 F05
R4
R3
1.23V
REFERENCE
V
IN
–
+
Figure 4. LTC1474/LTC1475 Adjustable Configuration
Figure 5. Low Battery Comparator
Low Battery Comparator
The LTC1474/LTC1475 have an on-chip low battery com-
parator that can be used to sense a low battery condition
when implemented as shown in Figure 5. The resistive
divider R3/R4 sets the comparator trip point as follows:
V
R
R
TRIP
=+
123 1
4
3
.
The divided down voltage at the LBI pin is compared to the
internal 1.23V reference. When V
LBI
< 1.23V, the LBO
output sinks current. The low battery comparator is active
all the time, even during shutdown mode.
11
LTC1474/LTC1475
APPLICATIONS INFORMATION
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LTC1475 Pushbutton On/Off and
Microprocessor Interface
The LTC1475 provides pushbutton control of power on/off
for use with handheld products. A momentary ground on
the ON pin sets an internal S/R latch to run mode while a
momentary ground on the LBI/OFF pin resets the latch to
shutdown mode. See Figure 6 for a comparsion of on/off
operation of the LTC1474 and LTC1475 and Figure 7 for a
comparison of the circuit implementations.
In the LTC1475, the LBI/OFF pin has a dual function as
both the shutdown control pin and the low battery com-
parator input. Since the “OFF” pushbutton is normally
open, it does not affect the normal operation of the low
battery comparator. In the unpressed state, the LBI/OFF
input is the divided down input voltage from the resistive
divider to the internal low battery comparator and will
normally be above or just below the trip threshold of
1.23V. When shutdown mode is desired, the LBI/OFF pin
is pulled below the 0.7V threshold to invoke shutdown.
the depressed switch state is detected by the microcon-
troller through its input. The microcontroller then pulls the
LBI/OFF pin low with the connection to one of its ouputs.
With the LBI/OFF pin low, the LTC1475 powers down
turning the microcontroller off. Note that since the I/O pins
of most microcontrollers have a reversed bias diode
between input and supply, a blocking diode with less than
1µA leakage is necessary to prevent the powered down
microcontroller from pulling down on the ON pin.
Figure 19 in the Typical Applications section shows how to
use the low battery comparator to provide a low battery
lockout on the “ON” switch. The LBO output disconnects
the pushbutton from the ON pin when the comparator has
tripped, preventing the LTC1475 from attempting to start
up again until V
IN
is increased.
RUN
RUN SHUTDOWN RUN
RUN
1474/75 F06
SHUTDOWN
RUN
MODE
ON OVERRIDES LBI/OFF
WHILE ON IS LOW
LTC1474
LTC1475
ON
LBI/OFF
MODE
Figure 6. Comparison of LTC1474 and LTC1475
Run/Shutdown Operation
The ON pin has precedence over the LBI/OFF pin. As seen
in Figure 6, if both pins are grounded simultaneously, run
mode wins.
Figure 18 in the Typical Applications section shows an
example for the use of the LTC1475 to control on/off of a
microcontroller with a single pushbutton. With both the
microcontroller and LTC1475 off, depressing the
pushbutton grounds the LTC1475 ON pin and starts up the
LTC1475 regulator which then powers up the microcon-
troller. When the pushbutton is depressed a second time,
1474/75 F07
RUN
ON
OFF
RUN
ON
LBI/OFF
100k
LTC1474
LTC1475
100k
V
IN
Figure 7. Simplified Implementation of
LTC1474 and LTC1475 On/Off
Absolute Maximum Ratings and Latchup Prevention
The absolute maximum ratings specify that SW (Pin 5) can
never exceed V
IN
(Pin 7) by more than 0.3V. Normally this
situation should never occur. It could, however, if the
output is held up while the supply is pulled down. A
condition where this could potentially occur is when a
battery is supplying power to an LTC1474 or LTC1475
regulator and also to one or more loads in parallel with the
the regulator’s V
IN
. If the battery is disconnected while the
LTC1474 or LTC1475 regulator is supplying a light load
and one of the parallel circuits is a heavy load, the input
capacitor of the LTC1474 or LTC1475 regulator could be
pulled down faster than the output capacitor, causing the
absolute maximum ratings to be exceeded. The result is
often a latchup which can be destructive if V
IN
is reapplied.
Battery disconnect is possible as a result of mechanical
stress, bad battery contacts or use of a lithium-ion battery
12
LTC1474/LTC1475
APPLICATIONS INFORMATION
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Figure 8. Preventing Absolute Maximum
Ratings from Being Exceeded
1474/75 F08
V
IN
V
OUT
LATCHUP
PROTECTION
SCHOTTKY
SW
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
IN
pins as shown in Figure 8. The diode will normally be
reverse biased unless V
IN
is pulled below V
OUT
at which
time the diode will clamp the (V
OUT
– V
IN
) potential to less
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
OUT
and ground with a value low
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:
T
R
= P • θ
JA
where P is the power dissipated by the regulator and θ
JA
is the thermal resistance from the junction of the die to the
ambient temperature.
The junction temperature is given by:
T
J
= T
A
+ T
R
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 3.5Ω. Therefore, power dissi-
pated by the part is:
P = I
2
• R
DS(ON)
= 0.315W
For the MSOP package, the θ
JA
is 150°C/W. Thus the
junction temperature of the regulator is:
T
J
= 70°C + (0.315)(150) = 117°C
which is near the maximum junction temperature of 125
o
C.
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
(Pin 5)?
2. Is the 0.1µF input decoupling capacitor
closely
con-
nected between V
IN
(Pin 7) and ground (Pin 4)? This
capacitor carries the high frequency peak currents.
3. When using adjustable version, is the resistive divider
closely connected to the (+) and (–) plates of C
OUT
with
a 10pF capacitor connected across R2?
4. Is the 1000pF decoupling capacitor for the current
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.
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LTC1475IS8#PBF

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Low IQ Push-Button Stepdn DC/DC Conv.
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