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LM3103 데이터시트(PDF) 11 Page - Texas Instruments

부품명 LM3103
상세설명  SIMPLE SWITCHER짰 Synchronous 1MHz 0.75A Step-Down Voltage Regulator
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LM3103 데이터시트(HTML) 11 Page - Texas Instruments

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8.3 x 10
-11
VIN(MAX) x 100 ns
RON t
LM3103
www.ti.com
SNVS523E – SEPTEMBER 2007 – REVISED NOVEMBER 2009
N-Channel MOSFET and Driver
The LM3103 integrates an N-Channel main MOSFET and an associated floating high voltage main MOSFET
gate driver. The gate drive circuit works in conjunction with an external bootstrap capacitor CBST and an internal
high voltage diode. CBST connected between the BST and SW pins powers the main MOSFET gate driver during
the main MOSFET on-time. During each off-time, the voltage of the SW pin falls to approximately -1V, and CBST
charges from VCC through the internal diode. The minimum off-time of 240 ns provides enough time for charging
CBST in each cycle.
Soft-Start
The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing
startup stresses and current surges. Upon turn-on, after VCC reaches the under-voltage threshold and a 180 µs
fixed delay, a 70 µA internal current source charges an external capacitor CSS connecting to the SS pin. The
ramping voltage at the SS pin (and the non-inverting input of the regulation comparator as well) ramps up the
output voltage VOUT in a controlled manner. An internal switch grounds the SS pin if any of the following three
cases happen: (i) VCC is below the under-voltage lockout threshold; (ii) a thermal shutdown occurs; or (iii) the EN
pin is grounded. Alternatively, the output voltage can be shut off by connecting the SS pin to the ground using an
external switch. Releasing the switch allows the voltage of the SS pin to ramp up and the output voltage to return
to normal. The shutdown configuration is shown in Figure 22.
Figure 22. Alternate Shutdown Implementation
Thermal Protection
The junction temperature of the LM3103 should not exceed the maximum limit. Thermal protection is
implemented by an internal Thermal Shutdown circuit, which activates (typically) at 165°C to make the controller
enter a low power reset state by disabling the main MOSFET, disabling the on-timer, and grounding the SS pin.
Thermal protection helps prevent catastrophic failures from accidental device overheating. When the junction
temperature falls back below 145°C (typical hysteresis = 20°C), the SS pin is released and normal operation
resumes.
Applications Information
EXTERNAL COMPONENTS
The following guidelines can be used to select external components.
RFB1 and RFB2 : These resistors should be chosen from standard values in the range of 1.0 kΩ to 10 kΩ,
satisfying the following ratio:
RFB1/RFB2 = (VOUT/0.6V) - 1
(7)
For VOUT = 0.6V, the FB pin can be connected to the output directly with a pre-load resistor drawing more than
20 µA. This is because the converter operation needs a minimum inductor current ripple to maintain good
regulation when no load is connected.
RON: Equation 2 can be used to select RON if a desired operating frequency is selected. But the minimum value
of RON is determined by the minimum on-time. It can be calculated as follows:
(8)
Copyright © 2007–2009, Texas Instruments Incorporated
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