DC924A - Demo Manual

DEMO CIRCUIT 924
LTC3772B
QUICK START
GUIDE
LTC3772B
No Rsense Constant Frequency StepDown DC/DC Controller in 3x2 DFN
DESCRIPTION
Demonstration circuit 924 is a No RSENSE™ constant frequency step-down DC/DC converter featuring the
LTC3772B Controller in a 3mm×2mm DFN package. The
demo circuit generates 2A output current at one of the
three output voltages: 1.8V, 2.5V or 3.3V. The output
voltage is selectable by JP1. Higher output current is
made possible using optional components. The exclusive
use of surface mount components results in a highly efficient application in a very small board space. The demo
circuit highlights the capabilities of the LTC3772B, which
uses current mode architecture to drive an external Pchannel power MOSFET. This results in fast loop dynamics and transient response. The ON resistance of the
MOSFET is used to sense the switch current. No additional sense resistor is needed. The LTC3772B’s internal
soft-start reduces the inrush current during start up. To
maximize the runtime from a battery source, the MOSFET
is turned on continuously in dropout (100% duty cycle).
The peak current sense voltage can be configured into
three different voltage thresholds for different loads. If
the output is shorted to ground, the frequency is folded
back to prevent current runaway. 550KHz constant frequency operation makes the LTC3772B ideal for low
noise applications.
The LTC3772B datasheet gives a complete description of
the part, operation and application information. The datasheet must be read in conjunction with this quick start
guide for demo circuit 924.
Design files for this circuit board are available. Call
the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
PERFORMANCE SUMMARY
Specifications are at TA = 25°C
SYMBOL
IQ
VIN
PARAMETER
Input DC Current in Shutdown Mode
Input Voltage
CONDITIONS
VITH/RUN = 0V, VIN = 4.2V
VOUT = 1.8V
VOUT = 2.5V
VOUT = 3.3V
VOUT
Output Voltage
ILOAD = 2A, JP1 on 1.8V position
ILOAD = 2A, JP1 on 2.5V position
ILOAD = 2A, JP1 on 3.3V position
IOUT(MAX) Maximum Output Current
EFF
Efficiency
VPP
F
OUTPUT VOLTAGE RIPPLE
Oscillator Frequency
VIN = 3V, VOUT = 1.8V, IOUT = 2A
VIN = 3V, VOUT = 2.5V, IOUT = 2A
VIN = 4V, VOUT = 3.3V, IOUT = 2A
VIN = 8V, VOUT = 3.3V, IOUT = 2A
VFB = 0.8V
MIN
3
3.3
3.9
1.728
2.4
3.168
2
500
TYP
8
MAX
20
8*
8*
1.8
2.5
3.3
1.872
2.6
3.432
UNITS
uA
V
8*
V
A
86%
91%
94%
22
550
650
mV
KHz
*: The maximum VIN is limited by the MOSFET gate voltage. The maximum VIN of LTC3772B is rated at 9.8V.
1
LTC3772B
QUICK START PROCEDURE
Demonstration circuit 924 is easy to set up to evaluate the
performance of the LTC3772B. Refer to Figure 1 for
proper measurement equipment setup and follow the
procedure below:
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.
4.
Check for the proper output voltages per the JP1 setup.
Note. If there is no output, temporarily disconnect the load to make sure
that the load is not set too high.
5.
Once the proper output voltages are established, adjust
the loads within the operating range and observe the
output voltage regulation, ripple voltage, efficiency and
other parameters.
6.
To evaluate output current higher than 2A, the footprints on the bottom side of the board can be used. Refer to the datasheet for more application examples.
One of three positions (1.8V, 2.5V or 3.3V)
With power off, connect the input power supply to VIN
and GND and the load to VOUT and GND.
Turn on the power at the input.
Note. Make sure that the input voltage does not exceed 8V.
Place jumpers in the following positions:
JP1
2.
3.
Figure 1. Proper Measurement Equipment Setup
GND
VIN
Figure 2. Measuring Input or Output Ripple
2
LTC3772B
3
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