DC1570A - Demo Manual

DEMO CIRCUIT 1570A-A/B
LT3663EDCB-3.3/5.0
QUICK
START GUIDE
LT3663EDCB-3.3/5.0
1.2A, 1MHz Step-Down
Switching Regulator with
Output Current Limit
DESCRIPTION
Demonstration Circuit 1570 is a 1.0MHz 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 LT3663-3.3/5.0 to a variety of input
sources, including unregulated 12V wall adapters, 24V
industrial supplies, and automotive power. The
LT3663-3.3/5.0 includes a low current shutdown
mode, input overvoltage lockout and thermal shutdown. The LT3663EDCB-3.3/5.0 is available in an 8lead (2mm × 3mm) DFN surface mount package with
exposed pad.
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.
TABLE1
PERFORMANCE SUMMARY Specifications are at TA = 25°C
SYMBOL
VIN
VOUTA
VOUTB
ILIM
PARAMETER
Input Voltage Range
Output Voltage Range
Output Voltage Range
Current Limit
CONDITIONS
Iout 0mA to ILIM
Iout 0mA to ILIM
Selectable ±20%
MIN
7.5
3.24
4.9
0.6
to
to
to
to
MAX
36.0
3.36
5.1
1.2
UNITS
V
V
V
A
OPERATING PRINCIPLE
Refer to the block diagram within the LT3663-3.3/5.0
data sheet for its operating principle.
The LT3663-3.3/5.0 is a constant frequency, current
mode step-down regulator. A switch cycle is initiated
when the 1MHz 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 internal resistor divider tied to the VOUT
pin and servos the VC node. If the error amplifier’s
output increases, more current is delivered to the out-
put; if it decreases, less current is delivered. An active
clamp (not shown) on the VC node provides current
limit. The LT3663-3.3/5.0 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.
1
LT3663EDCB-3.3/5.0
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
LT3663-3.3/5.0 can safely sustain input voltages up to
60V. Note that while switching is disabled the output will
start to discharge.
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.
Output current limiting is provided via the servo action of
an amplifier. It compares the voltage across an inductor
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.
Follow the procedure below:
1. Jumper, PS and LOAD settings to start:
JP1 = Run
PS1 = OFF
JP2 = 1
LOAD1 = OFF
JP3 = 1
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 the 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 in Table 1.
4. Set LOAD1 to 1.0A. Verify voltage on VOUT in
Table 1 and ripple voltage of <50mV.
5. Increase LOAD1 current until VOUT drops below the minimum voltage in Table 1. Verify
LOAD1 current is between 0.96A and 1.44A.
Reduce LOAD1 current to 120mA.
6. Set JP2 to 1 and JP3 to 0. Increase LOAD1
current until VOUT drops below the minimum
2
voltage in Table 1. Verify LOAD1 current is between 800mA and 1.12A. Reduce LOAD1 current to 120mA.
7. Set JP2 to 0 and JP3 to 1. Increase LOAD1
current until VOUT drops below the minimum
voltage in Table 1. Verify LOAD1 current is between 640mA and 960mA. Reduce LOAD1
current to 120mA.
8. Set JP2 to 0 and JP3 to 0. Increase LOAD1
current until VOUT drops below the minimum
voltage in Table 1. Verify LOAD1 current is between 480mA and 720mA. Reduce LOAD1
current to 120mA.
9. Increase PS1 to 36V and verify voltage on
VOUT in Table 1.
10. Increase PS1 to 40V and verify voltage on
VOUT of <500mV.
11. Decrease PS1 to 30V and verify voltage on
VOUT in Table 1.
12. Turn off PS1 and LOAD1.
LT3663EDCB-3.3/5.0
Figure 1: Proper Measurement Equipment Setup
3
LT3663EDCB-3.3/5.0
Figure 2: Schematic diagram
4
LT3663EDCB-3.3/5.0
Bill of Materials
5
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