DC1419A - Demo Manual

DEMO CIRCUIT 1419A
LT3663EDCB
QUICK START
GUIDE
LT3663EDCB
1.2A, 1.5MHz Step-Down
Switching Regulator with
Output Current Limit
DESCRIPTION
Demonstration Circuit 1419 is a 1.5MHz current mode
step-down switching regulator with programmable
output current limit. The current limit accurately controls the system power dissipation and reduces the
size of the power path components.
The wide operating input voltage range of 7.5V to 36V
(60V maximum) suits the LT3663 to a variety of input
sources, including unregulated 12V wall adapters, 24V
industrial supplies, and automotive power.
The LT3663 includes a low current shutdown mode,
input overvoltage lockout and thermal shutdown.
The LT3663EDCB is available in an 8-lead (2mm ×
3mm) DFN surface mount package with exposed pad.
Design files for this circuit board are available. Call
the LTC factory.
L, LTC, LTM, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered
trademarks of Linear Technology Corporation. Adaptive Power, C-Load, DirectSense, Easy
Drive, FilterCAD, Hot Swap, LinearView, μModule, Micropower SwitcherCAD, Multimode
Dimming, No Latency ΔΣ, No Latency Delta-Sigma, No RSENSE, Operational Filter, PanelProtect,
PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT,
UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names
may be trademarks of the companies that manufacture the products.
PERFORMANCE SUMMARY Specifications are at TA = 25°C
SYMBOL
VIN
VOUT
ILIM
PARAMETER
Input Voltage Range
Output Voltage Range
Current Limit
CONDITIONS
Iou1 0mA to ILIM
MIN
7.5
3.3
0.6
to
or
MAX
36.0
5.0
1.2
UNITS
V
V
A
OPERATING PRINCIPLE
Refer to the block diagram within the LT3663 data
sheet for its operating principle.
The LT3663 is a constant frequency, current mode step
down regulator. A switch cycle is initiated when the
1.5MHz oscillator enables the RS flip flop, turning on an
internal power switch, Q1. An amplifier and comparator
monitor the current flowing between the VIN and SW
pins, turning the switch off when the current reaches a
level determined by the voltage at node VC. The error
amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the VC
node. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp (not shown) on the VC
node provides current limit. The LT3663 is internally
compensated with a pole zero combination on the output
of the gm amplifier.
An external capacitor and internal diode are used to generate a voltage at the BOOST pin that is higher than the
input supply. This allows the driver to fully saturate the
internal bipolar NPN power switch for efficient operation.
The switch driver operates from either VIN or BOOST to
ensure startup.
An internal regulator provides power to the control circuitry. This regulator includes an input under-voltage and
overvoltage protection which disable switching action
when VIN is out of range. When switching is disabled,
the LT3663 can safely sustain input voltages up to 60V.
1
LT3663EDCB
Note that while switching is disabled the output will start
to discharge.
Output current limiting is provided via the servo action of
an amplifier. It compares the voltage across an inductor
current sense resistor, RSENSE2, and compares it to a
voltage programmed by external resistor R1 on the ILIM
pin. A capacitor averages the inductor ripple current. If
the averaged inductor current exceeds the programmed
value then the VC voltage is pulled low, reducing the current in the regulator. The output current limit circuit allows for a lower current rated inductor and diode and
provides better control of system power dissipation.
QUICK START PROCEDURE
Using short twisted pair leads for any power connections, with all loads and power supplies off, refer
to Figure 1 for the proper measurement and equipment setup.
6. Set JP2 to 1 and JP3 to 0. Increase LOAD1
current until VOUT drops below 4.9V. Verify
LOAD1 current is between 900mA and 1.1A.
Reduce LOAD1 current to 120mA.
Follow the procedure below:
7. Set JP2 to 0 and JP3 to 1. Increase LOAD1
current until VOUT drops below 4.9V. Verify
LOAD1 current is between 700mA and
900mA. Reduce LOAD1 current to 120mA.
1. Jumper, PS and LOAD 1ettings to start:
JP1 = Run
JP4 = 5.0V
JP2 = 1
PS1 = OFF
JP3 = 1
LOAD1 = OFF
2. Turn on PS1 and slowly increase voltage to
5.5V while monitoring the input current. If the
current remains less than 50mA, increase PS1
until output turns on. Verify input voltage
UVLO of 6.5V to 7.5V.
3. Increase PS1 to 12V and set LOAD1 to
120mA. Verify voltage on VOUT of 4.85V to
5.15V.
4. Set LOAD1 to 1.0A. Verify voltage on VOUT of
4.85V to 5.15V and ripple voltage of <50mV.
5. Increase LOAD1 current until VOUT drops below 4.9V. Verify LOAD1 current is between
1.1A and 1.4A. Reduce LOAD1 current to
120mA.
2
8. Set JP2 to 0 and JP3 to 0. Increase LOAD1
current until VOUT drops below 4.9V. Verify
LOAD1 current is between 500mA and
700mA. Reduce LOAD1 current to 120mA.
9. Set JP2 to 1, JP3 to 1 and JP4 to 3.3V. Verify
voltage on VOUT of 3.2V to 3.4V
10. Set LOAD1 to 1.0A. Verify voltage on VOUT of
3.2V to 3.4V and ripple voltage of <50mV.
11. Increase PS1 to 36V and verify voltage on
VOUT of 3.2V to 3.4V.
12. Increase PS1 to 40V and verify voltage on
VOUT of 500mV.
13. Decrease PS1 to 30V and verify voltage on
VOUT of 3.2V to 3.4V.
14. Turn off PS1 and Load 1.
LT3663EDCB
Figure 1. Proper Measurement Equipment Setup
3
LT3663EDCB
Figure 2: Schematic diagram
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LT3663EDCB
Bill of Materials
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