VOUT

1.3 x 10

-10 x R

ON

fSW =

(VIN ± VOUT) x RON

2

VOUT (VIN - 1) x L x 1.18 x 10

20 x I

OUT

fSW =

12

LM3102

SNVS515I – SEPTEMBER 2007 – REVISED JANUARY 2018

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Copyright © 2007–2018, Texas Instruments Incorporated

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 because 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 = 0.8V x (RFB1 + RFB2)/RFB2

(3)

7.3.2 Start-Up Regulator (VCC)

A startup regulator is integrated within the LM3102. 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 65 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 680 nF. When the voltage on the VCC pin is higher than the UVLO threshold of 3.75 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 (

≊3.7 V). If VIN is less than ≊4.0 V, the regulator shuts off and VCC goes to zero.

7.3.3 Regulation Comparator

The feedback voltage at the FB pin is compared to a 0.8-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 0.8 V. The main

MOSFET stays on for the ON-time, causing the output voltage and consequently the voltage of the FB pin to rise

above 0.8 V. After the ON-time period, the main MOSFET stays off until the voltage of the FB pin falls below 0.8

V again. Bias current at the FB pin is nominally 5 nA.

7.3.4 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.

7.3.5 Overvoltage Comparator

The voltage at the FB pin is compared to a 0.92-V internal reference. If the voltage rises above 0.92 V, the ON-

time is immediately terminated. This condition is known as overvoltage 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 0.92 V. The synchronous MOSFET will stay on to discharge the inductor until

the inductor current reduces to zero, and then switch off.