LTC1626 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1626' PDF : 12 Pages View PDF
LTC1626
APPLICATIONS INFORMATION
ESRCOUT < 2RSENSE
To avoid overheating, the output capacitor must be sized
to handle the ripple current generated by the inductor. The
worst-case RMS ripple current in the output capacitor is
given by:
IRMS < 150mV/2RSENSE (ARMS)
Generally, once the ESR requirements for COUT have been
met, the RMS current rating far exceeds the IRIPPLE
requirement.
In some surface mount applications, multiple capacitors
may have to be paralleled to meet the capacitance, ESR or
RMS current handling requirement of the application.
Aluminum electrolyte and dry tantalum capacitors are
both available in surface mount configurations. In the case
of tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. An excellent choice is
the AVX TPS series of surface mount tantalums, available
in case heights ranging from 2mm to 4mm. Other capaci-
tor types include Sanyo OS-CON, Nichicon PL series and
Sprague 595D series. Consult the manufacturer for other
specific recommendations.
When the capacitance of COUT is made too small, the
output ripple at low frequencies will be large enough to trip
the voltage comparator. This causes Burst Mode opera-
tion to be activated when the LTC1626 would normally be
in continuous mode operation. The effect will be most
pronounced with low RSENSE values and can be improved
at higher frequencies.
Low-Battery Detection
The low-battery detector senses the input voltage through
an external resistive divider. This divided voltage connects
to the (–) input of a voltage comparator (LBI) and is
compared to an internal 1.25V reference voltage. Neglect-
ing LBI input bias current, the following expression is used
for setting the trip voltage threshold:
VLB _
TRIP
=
1.25
1
+
R4
R3
The LBO is an N-channel open drain that goes low when
the battery voltage drops below the threshold voltage. In
shutdown, the comparator is disabled and LBO is in the
high impedance state. Figure 4 is a schematic diagram
detailing the low-battery comparator connection and op-
eration.
VIN
R4
1%
1.25V
LBI
LTC1626
+
–
CFILTER
R3
0.01µF
1%
LBO
1626 F04
Figure 4. Low-Battery Comparator
Setting the Output Voltage
The LTC1626 develops a 1.25V reference voltage between
the feedback pin VFB and the signal ground as shown in
Figure 5. By selecting resistor R1, a constant current is
caused to flow through R1 and R2 which sets the desired
output voltage. The regulated output voltage is deter-
mined by:
VOUT
=
1.251+
R2
R1
R1 should be ≤ 10k to ensure that sufficient current flows
through the divider to maintain accuracy and to provide a
minimum load for the regulator output at elevated
temperatures. (See Switch Leakage Current curve in Typi-
cal Performance Characteristics section.)
To prevent stray pickup, a 100pF capacitor is suggested
across R1, located close to the LTC1626.
LTC1626 VFB
SGND
VOUT
R2
1%
R1
100pF 10k
1%
1626 F05
Figure 5. Setting the Output Voltage
8
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