DC840A - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 840
HIGH EFFICIENCY, LOW PROFILE, SYNCHRONOUS BUCK/LDO REGULATOR
LTC3448
DESCRIPTION
Demonstration circuit DC840 is a step-down converter,
using the LTC3448 monolithic synchronous buck regulator. The DC840 has an input voltage range of 2.5V to
5.5V, and is capable of delivering up to 600mA of output
current. The output voltage of DC840 can be set as low
as 0.6V, the reference voltage of the LTC3448. The
DC840 is especially made for noise sensitive applications, due to a unique design that allows the LTC3448 to
automatically switch – in auto mode - from a switching
regulator into a linear regulator at low output currents
(In switch-mode, the LTC3448 operates in pulseskipping mode at medium-low output currents). This
operation of the LTC3448 maintains low output ripple
voltage under all load conditions. There is a mode pin to
select auto mode, ON (linear regulator only), or OFF
(switching regulator only).
The DC840 is a high efficiency circuit - over 90%, and
during shutdown, the DC840 consumes less than 1uA
typically. It consumes only 32uA of quiescent current in
linear regulation operation. Because of the frequencyselect capability of the LTC3448, the DC840 can operate
at fixed frequencies of 1.5 MHz or 2.25 MHz, allowing
the exclusive use of low profile surface mount components. The DC840 can also be clocked by an external
oscillator, due to a phase-lock loop circuit in the
LTC3448. These features, plus the LTC3448 coming in
low profile 8-Lead DFN or MSOP packages, make the
DC840 an ideal circuit for use in battery-powered, handheld applications.
Design files for this circuit board are available. Call
the LTC factory.
LTC is a trademark of Linear Technology Corporation
Table 1. Performance Summary (TA = 25°C)
PARAMETER
Minimum Input Voltage
CONDITION
VALUE
2.5V
Maximum Input Voltage
Output Voltage VOUT
Typical Output Ripple VOUT
Output Regulation
5.5V
VIN = 2.5V to 5.5V, IOUT1 = 0A to 600 mA
1.2V ±4%
VIN = 5V, IOUT1 = 600 mA
20mVP–P
Line
±1%
Load
±1%
Nominal Switching Frequency
1.5 or 2.25 MHz
QUICK START PROCEDURE
DC840 is easy to set up to evaluate the performance
of the LTC3448. Refer to Figure 1. for proper measurement equipment setup and follow the procedure
outlined below. Before proceeding to test, insert
jumper JP1 shunt into the off (lower) position, connecting the RUN pin to ground (GND), which shuts
down the circuit.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 840
HIGH EFFICIENCY, LOW PROFILE, SYNCHRONOUS BUCK/LDO REGULATOR
NOTE: When measuring the input or output voltage
ripple, care must be taken to avoid a long ground lead
on the oscilloscope probe. Measure the input or output voltage ripple by touching the probe tip directly
across the VIN or VOUT and GND terminals. See
Figure 2 for proper scope probe technique.
1.
2.
Connect the input power supply and the load to the
board. Do not hot-plug Vin or increase Vin over
the rated maximum supply voltage of 5.5V, or the
part may be damaged.
Insert shunts into the 1.5MHz position of jumper
JP2 and into the jumper JP7, the 1.8V output voltage option. Also, insert the jumper JP3 shunt into
the free-running position and the jumper JP4 shunt
into the AUTO position.
6.
Measure the output ripple voltage at any output
current level; it usually will measure less than
20mVAC.
7.
Observe the voltage waveform at the switch node
(pin 5 of the IC). Verify the switching frequency is
between 1.2MHz and 1.8MHz (T = 0.833us and
0.556us), and that the switch node waveform is
rectangular in shape.
8.
Still observing the switch node, from no load, increase the output current until switching operation
is observed. This should occur at approx. 11mA.
Also observe the switch node as the load is decreased to 0A. The LTC3448 should turn into a linear regulator at about 2.5mA (hysteresis).
9.
Insert jumper JP1 into the OFF (lower) position,
and insert shunts into the other voltage options,
1.2V or 1.5V, and the other frequency operation,
2.25MHz. Re-insert the jumper JP1 shunt into the
ON (upper) position. The DC840 should perform
the same as in the 1.8Vout evaluation.
3.
Apply 3.3V at VIN. Measure VOUT; it should read
0V. If desired, one can measure the shutdown
supply current at this point. The supply current
will be approximately 1uA in shutdown.
4.
Turn on the circuit by inserting the shunt in jumper
JP1 into the ON (upper) position. The output voltage should be regulating. Measure VOUT - it
should measure 1.8V +/- 2%.
The options of the mode feature – AUTO, ON, and
OFF can also be evaluated now: On for linear operation only, and OFF for switching operation only.
Vary the input voltage from 2.5V to 5.5V and the
load current from 0 to 600mA. Vout should read
between 1.8V +/- 4%.
When finished, turn off the circuit (connecting the
RUN pin to ground) by inserting the shunt in jumper
JP1 into the OFF (lower) position.
5.
Warning - if the power for the demo board is carried in long leads, the input voltage at the part could “ring”. To
eliminate this, insert a small tantalum capacitor (for instance, an AVX part # TAJW476M010R) on the pads between the input power and return terminals on the bottom of the demo board. The (greater) ESR of the tantalum will dampen the (possible) ringing voltage due to the use of long input leads. On a normal, typical PCB,
with short traces, the capacitor is not needed.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 840
HIGH EFFICIENCY, LOW PROFILE, SYNCHRONOUS BUCK/LDO REGULATOR
Figure 1. Proper Measurement Equipment Setup
Figure 2. Measuring Input or Output Ripple
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 840
HIGH EFFICIENCY, LOW PROFILE, SYNCHRONOUS BUCK/LDO REGULATOR
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