OMO ElectronicOMO Electronic
Expand menu
Hello, Sign inMy Account
0 Cart

LT1934IDCB-1#TRMPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
LT1934/LT1934-1
10
1934fe
capacitor is subject to large surge currents if the LT1934
circuit is connected to a low impedance supply, and that
some electrolytic capacitors (in particular tantalum) must
be specifi ed for such use.
Output Capacitor and Output Ripple
The output capacitor fi lters the inductor’s ripple current and
stores energy to satisfy the load current when the LT1934
is quiescent. In order to keep output voltage ripple low, the
impedance of the capacitor must be low at the LT1934’s
switching frequency. The capacitor’s equivalent series
resistance (ESR) determines this impedance. Choose one
with low ESR intended for use in switching regulators. The
contribution to ripple voltage due to the ESR is approxi-
mately I
LIM
• ESR. ESR should be less than ~150mΩ for
the LT1934 and less than ~500mΩ for the LT1934-1.
The value of the output capacitor must be large enough
to accept the energy stored in the inductor without a large
change in output voltage. Setting this voltage step equal to
1% of the output voltage, the output capacitor must be:
C
OUT
> 50 • L • (I
LIM
/V
OUT
)
2
For example, an LT1934 producing 3.3V with L = 47μH
requires 33μF. This value can be relaxed if small circuit
size is more important than low output ripple.
Sanyo’s POSCAP series in B-case and C-case sizes
provides very good performance in a small package for
the LT1934. Similar performance in traditional tantalum
capacitors requires a larger package (C- or D-case).
APPLICATIONS INFORMATION
The LT1934-1, with its lower switch current, can use a
B-case tantalum capacitor.
With a high quality capacitor fi ltering the ripple current
from the inductor, the output voltage ripple is determined
by the hysteresis and delay in the LT1934’s feedback
comparator. This ripple can be reduced further by adding
a small (typically 10pF) phase lead capacitor between the
output and the feedback pin.
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT1934.
Not all ceramic capacitors are suitable. X5R and X7R
types are stable over temperature and applied voltage
and give dependable service. Other types (Y5V and Z5U)
have very large temperature and voltage coeffi cients of
capacitance. In the application circuit they may have only
a small fraction of their nominal capacitance and voltage
ripple may be much larger than expected.
Ceramic capacitors are piezoelectric. The LT1934’s switch-
ing frequency depends on the load current, and at light
loads the LT1934 can excite the ceramic capacitor at audio
frequencies, generating audible noise. If this is unaccept-
able, use a high performance electrolytic capacitor at the
output. The input capacitor can be a parallel combination
of a 2.2μF ceramic capacitor and a low cost electrolytic
capacitor. The level of noise produced by the LT1934-1
Table 3. Capacitor Vendors
VENDOR PHONE URL PART SERIES COMMENTS
Panasonic (714) 373-7366 www.panasonic.com Ceramic,
Polymer,
Tantalum
EEF Series
Kemet (864) 963-6300 www.kemet.com Ceramic,
Tantalum T494, T495
Sanyo (408) 749-9714 www.sanyovideo.com Ceramic,
Polymer,
Tantalum
POSCAP
Murata (404) 436-1300 www.murata.com Ceramic,
AVX www.avxcorp.com Ceramic,
Tantalum
TPS Series
Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Ceramic
LT1934/LT1934-1
11
1934fe
APPLICATIONS INFORMATION
when used with ceramic capacitors will be lower and may
be acceptable.
A fi nal precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT1934. A ceramic
input capacitor combined with trace or cable inductance
forms a high quality (under damped) tank circuit. If the
LT1934 circuit is plugged into a live supply, the input volt-
age can ring to twice its nominal value, possibly exceeding
the LT1934’s rating. This situation is easily avoided; see
the Hot Plugging Safely section.
Catch Diode
A 0.5A Schottky diode is recommended for the catch
diode, D1. The diode must have a reverse voltage rating
equal to or greater than the maximum input voltage. The
ON Semiconductor MBR0540 is a good choice; it is rated
for 0.5A forward current and a maximum reverse voltage
of 40V.
Schottky diodes with lower reverse voltage ratings usu-
ally have a lower forward drop and may result in higher
effi ciency with moderate to high load currents. However,
these diodes also have higher leakage currents. This leakage
current mimics a load current at the output and can raise
the quiescent current of the LT1934 circuit, especially at
elevated temperatures.
BOOST Pin Considerations
Capacitor C3 and diode D2 are used to generate a boost
voltage that is higher than the input voltage. In most cases
a 0.1μF capacitor and fast switching diode (such as the
1N4148 or 1N914) will work well. Figure 2 shows two
ways to arrange the boost circuit. The BOOST pin must
be more than 2.5V above the SW pin for best effi ciency.
For outputs of 3.3V and above, the standard circuit (Fig-
ure 2a) is best. For outputs between 2.8V and 3V, use a
0.22μF capacitor and a small Schottky diode (such as the
BAT-54). For lower output voltages the boost diode can be
tied to the input (Figure 2b). The circuit in Figure 2a is more
effi cient because the BOOST pin current comes from a lower
voltage source. You must also be sure that the maximum
voltage rating of the BOOST pin is not exceeded.
The minimum operating voltage of an LT1934 applica-
tion is limited by the undervoltage lockout (~3V) and by
the maximum duty cycle as outlined above. For proper
start-up, the minimum input voltage is also limited by the
boost circuit. If the input voltage is ramped slowly, or the
LT1934 is turned on with its SHDN pin when the output
is already in regulation, then the boost capacitor may not
be fully charged. Because the boost capacitor is charged
with the energy stored in the inductor, the circuit will rely
on some minimum load current to get the boost circuit
running properly. This minimum load will depend on input
and output voltages, and on the arrangement of the boost
circuit. The minimum load generally goes to zero once the
circuit has started. Figure 3 shows a plot of minimum load
to start and to run as a function of input voltage. In many
cases the discharged output capacitor will present a load
to the switcher which will allow it to start. The plots show
the worst-case situation where V
IN
is ramping very slowly.
Use a Schottky diode (such as the BAT-54) for the lowest
start-up voltage.
At light loads, the inductor current becomes discontinu-
ous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
300mV above V
OUT
. At higher load currents, the inductor
current is continuous and the duty cycle is limited by the
V
IN
BOOST
GND
SW
V
IN
LT1934
(2a)
D2
V
OUT
C3
V
BOOST
– V
SW
V
OUT
MAX V
BOOST
V
IN
+ V
OUT
V
IN
BOOST
GND
SW
V
IN
LT1934
(2b)
D2
1934 F02
V
OUT
C3
V
BOOST
– V
SW
V
IN
MAX V
BOOST
2V
IN
Figure 2. Two Circuits for Generating the Boost Voltage
LT1934/LT1934-1
12
1934fe
APPLICATIONS INFORMATION
maximum duty cycle of the LT1934, requiring a higher
input voltage to maintain regulation.
Shorted Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT1934 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT1934 is absent. This may occur in battery charging ap-
plications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT1934’s
output. If the V
IN
pin is allowed to fl oat and the SHDN pin
is held high (either by a logic signal or because it is tied
Figure 3. The Minimum Input Voltage Depends
on Output Voltage, Load Current and Boost Circuit
Minimum Input Voltage V
OUT
= 3.3V
Minimum Input Voltage V
OUT
= 5V
LOAD CURRENT (mA)
3.5
INPUT VOLTAGE (V)
4.0
4.5
5.0
5.5
6.0
0.1 10 100
1934 G12
3.0
1
LT1934
V
OUT
= 3.3V
T
A
= 25°C
BOOST DIODE TIED TO OUTPUT
V
IN
TO START
V
IN
TO RUN
LOAD CURRENT (mA)
5
INPUT VOLTAGE (V)
6
7
8
0.1 10 100
1934 G13
4
1
LT1934
V
OUT
= 5V
T
A
= 25°C
BOOST DIODE TIED TO OUTPUT
V
IN
TO START
V
IN
TO RUN
to V
IN
), then the LT1934’s internal circuitry will pull its
quiescent current through its SW pin. This is fi ne if your
system can tolerate a few mA in this state. If you ground
the SHDN pin, the SW pin current will drop to essentially
zero. However, if the V
IN
pin is grounded while the output
is held high, then parasitic diodes inside the LT1934 can
pull large currents from the output through the SW pin
and the V
IN
pin. Figure 4 shows a circuit that will run only
when the input voltage is present and that protects against
a shorted or reversed input.
Figure 4. Diode D4 Prevents a Shorted Input from Discharging
a Backup Battery Tied to the Output; It Also Protects the Circuit
from a Reversed Input. The LT1934 Runs Only When the Input
is Present
V
IN
BOOST
GND FB
SHDN SW
5
D4
V
IN
4
1
6
23
1M
100k
LT1934
1934 F07
V
OUT
BACKUP
D4: MBR0530
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 5 shows
the high current paths in the buck regulator circuit. Note
that large, switched currents fl ow in the power switch,
the catch diode (D1) and the input capacitor (C2). The
loop formed by these components should be as small as
possible. Furthermore, the system ground should be tied
to the regulator ground in only one place; this prevents
the switched current from injecting noise into the system
ground. These components, along with the inductor and
output capacitor, should be placed on the same side of
the circuit board, and their connections should be made
on that layer. Place a local, unbroken ground plane below
these components, and tie this ground plane to system
ground at one location, ideally at the ground terminal of
the output capacitor C1. Additionally, the SW and BOOST
nodes should be kept as small as possible. Finally, keep
the FB node as small as possible so that the ground pin
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LT1934IDCB-1#TRMPBF

Mfr. #:
Buy LT1934IDCB-1#TRMPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 90mA (Isw), Micropower Step-Down DC/DC in 2mm x 3mm DFN-6
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
Payment:
T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet