VIN
8.3 x 10-11 x RON
tON =
VOUT
8.3 x 10-11 x RON
fSW =
(VIN ± VOUT) x RON2
VOUT (VIN - 1) x L x 1.18 x 1020 x IOUT
fSW =
11
LM3103
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SNVS523G –SEPTEMBER 2007–REVISED JANUARY 2018
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Feature Description (continued)
The regulator will operate in the discontinuous conduction mode (DCM) at a light load, and the continuous
conduction mode (CCM) with a heavy load. In the DCM, the current through the inductor starts at zero and
ramps up to a peak during the on-time, and then ramps back to zero before the end of the off-time. It remains
zero and the load current is supplied entirely by the output capacitor. The next on-time period starts when the
voltage at the FB pin falls below the internal reference. The operating frequency in the DCM is lower and varies
larger with the load current as compared with the CCM. Conversion efficiency is maintained since conduction
loss and switching loss are reduced with the reduction in the load and the switching frequency respectively. The
operating frequency in the DCM can be calculated approximately as follows:
(1)
In the continuous conduction mode (CCM), the current flows through the inductor in the entire switching cycle,
and never reaches zero during the off-time. The operating frequency remains relatively constant with load and
line variations. The CCM operating frequency can be calculated approximately as follows:
(2)
The output voltage is set by two external resistors RFB1 and RFB2. The regulated output voltage is
VOUT = 0.6V x (RFB1 + RFB2)/RFB2 (3)
7.2.2 Startup Regulator (VCC)
A startup regulator is integrated within the LM3103. The input pin VIN can be connected directly to a line voltage
up to 42 V. The VCC output regulates at 6 V, and is current limited to 30 mA. Upon power up, the regulator
sources current into an external capacitor CVCC, which is connected to the VCC pin. For stability, CVCC must be at
least 1 µF. When the voltage on the VCC pin is higher than the under-voltage lock-out (UVLO) threshold of 3.7
V, the main MOSFET is enabled and the SS pin is released to allow the soft-start capacitor CSS to charge.
The minimum input voltage is determined by the dropout voltage of the regulator and the VCC UVLO falling
threshold (≊3.4 V). If VIN is less than ≊4.0 V, the regulator shuts off and VCC goes to zero.
7.2.3 Regulation Comparator
The feedback voltage at the FB pin is compared to a 0.6 V internal reference. In normal operation (the output
voltage is regulated), an on-time period is initiated when the voltage at the FB pin falls below 0.6 V. The main
MOSFET stays on for the programmed on-time, causing the output voltage to rise and consequently the voltage
of the FB pin to rise above 0.6 V. After the on-time period, the main MOSFET stays off until the voltage of the FB
pin falls below 0.6 V again. Bias current at the FB pin is nominally 1 nA.
7.2.4 Zero Coil Current Detect
The current of the synchronous MOSFET is monitored by a zero coil current detection circuit which inhibits the
synchronous MOSFET when its current reaches zero until the next on-time. This circuit enables the DCM
operation, which improves the efficiency at a light load.
7.2.5 Over-Voltage Comparator
The voltage at the FB pin is compared to a 0.68 V internal reference. If it rises above 0.68 V, the on-time is
immediately terminated. This condition is known as over-voltage protection (OVP). It can occur if the input
voltage or the output load changes suddenly. Once the OVP is activated, the main MOSFET remains off until the
voltage at the FB pin falls below 0.6 V. The synchronous MOSFET will stay on to discharge the inductor until the
inductor current reduces to zero and then switch off.
7.2.6 ON-Time Timer, Shutdown
The on-time of the LM3103 main MOSFET is determined by the resistor RON and the input voltage VIN. It is
calculated as follows:
(4)