8.3.1 Programming the Output Voltage

The output voltage is set by two external resistors (R_FB1,R_FB2). The regulated output voltage is calculated as follows:

Equation 5.

where

• R_FB2 is the top resistor connected between VOUT and FB
• R_FB1 is the bottom resistor connected between FB and GND

8.3.2 Regulation Comparator

The feedback voltage at FB is compared to the internal reference voltage of 0.6 V. In normal operation (the output voltage is regulated), an on-time period is initiated when the voltage at FB falls below 0.6 V. The high-side switch stays on for the on-time, causing the FB voltage to rise above 0.6 V. After the on-time period, the high-side switch stays off until the FB voltage falls below 0.6 V.

8.3.3 Overvoltage Comparator

The overvoltage comparator is provided to protect the output from overvoltage conditions due to sudden input line voltage changes or output loading changes. The overvoltage comparator continuously monitors the voltage at the FB pin and compares it to a 0.72 V internal reference. If the voltage at FB rises above 0.72 V, the on-time pulse is immediately terminated. This condition can occur if the input or the output load changes suddenly. Once the overvoltage protection is activated, the HG and LG signals remain off until the voltage at FB pin falls below 0.72 V.

8.3.4 Current Limit

Current limit detection occurs during the off-time by monitoring the current through the low-side switch using an external resistor, R_LIM. If during the off-time the current in the low-side switch exceeds the user defined current limit value, the next on-time cycle is immediately terminated. Current sensing is achieved by comparing the voltage across the low side FET with the voltage across the current limit set resistor R_LIM. If the voltage across R_LIM and the voltage across the low-side FET are equal then the current limit comparator will terminate the next on-time cycle.

The R_LIM value can be approximated as follows:

Equation 6.

Equation 7.

where

• I_OCL is the user-defined average output current limit value
• R_DS(ON)max is the resistance value of the low-side FET at the expected maximum FET junction temperature
• I_LIM-TH is an internal current supply of 85 µA typical

Figure 13 illustrates the inductor current waveform. During normal operation, the output current ripple is dictated by the switching of the FETs. The current through the low-side switch, I_valley, is sampled at the end of each switching cycle and compared to the current limit, I_CL, current. The valley current can be calculated as follows:

Equation 8.

where

• I_OUT is the average output current
• ΔI_L is the peak-to-peak inductor ripple current

If an overload condition occurs, the current through the low-side switch will increase which will cause the current limit comparator to trigger the logic to skip the next on-time cycle. The IC will then try to recover by checking the valley current during each off-time. If the valley current is greater than or equal to I_CL, then the IC will keep the low-side FET on and allow the inductor current to further decay.

Throughout the whole process, regardless of the load current, the on-time of the controller will stay constant and thereby the positive ripple current slope will remain constant. During each on-time the current ramps-up an amount equal to:

Equation 9.

The valley current limit feature prevents current runaway conditions due to propagation delays or inductor saturation because the inductor current is forced to decay following any overload conditions.

Current sensing is achieved by either a low value sense resistor in series with the low-side FET or by utilizing the R_DS(ON) of the low-side FET. The R_DS(ON) sensing method is the preferred choice for a more simplified design and lower costs. The R_DS(ON) value of a FET has a positive temperature coefficient and will increase in value as the temperature of the FET increases. The LM3150 controller will maintain a more stable current limit that is closer to the original value that was set by the user, by positively adjusting the I_LIM-TH value as the IC temperature increases. This does not provide an exact temperature compensation but allows for a more tightly controlled current limit when compared to traditional R_DS(ON) sensing methods when the R_DS(ON) value can change typically 140% from room to maximum temperature and cause other components to be over-designed. The temperature compensated I_LIM-TH is shown below where T_J is the die temperature of the LM3150 controller in Celsius:

To calculate the R_LIM value with temperature compensation, substitute Equation 10 into I_LIM-TH in Equation 7.

Figure 13. Inductor Current - Current Limit Operation

8.3.5 Short-Circuit Protection

The LM3150 controller will sense a short-circuit on the output by monitoring the output voltage. When the feedback voltage has fallen below 60% of the reference voltage, V_ref x 0.6 (≈ 0.36 V), short-circuit mode of operation will start. During short-circuit operation, the SS pin is discharged and the output voltage will fall to 0 V. The SS pin voltage, V_SS, is then ramped back up at the rate determined by the SS capacitor and I_SS until V_SS reaches 0.7 V. During this re-ramp phase, if the short-circuit fault is still present the output current will be equal to the set current limit. Once the soft-start voltage reaches 0.7 V, the output voltage is sensed again and if the V_FB is still below V_ref x 0.6 then the SS pin is discharged again and the cycle repeats until the short-circuit fault is removed.

8.3.6 Soft-Start

The soft-start (SS) feature allows the regulator to gradually reach a steady-state operating point, which reduces start-up stresses and current surges. At turnon, while VCC is below the undervoltage threshold, the SS pin is internally grounded and V_OUT is held at 0 V. The SS capacitor is used to slowly ramp V_FB from 0 V to 0.6 V. By changing the capacitor value, the duration of start-up can be changed accordingly. The start-up time can be calculated using the following equation:

Equation 11.

where

• t_SS is measured in seconds
• V_ref = 0.6 V
• I_SS is the soft-start pin source current, which is typically 7.7 µA (refer to Electrical Characteristics)

An internal switch grounds the SS pin if VCC is below the undervoltage lockout threshold, if a thermal shutdown occurs, or if the EN pin is grounded. By using an externally controlled switch, the output voltage can be shut off by grounding the SS pin.

During startup the LM3150 controller will operate in diode emulation mode, where the low-side gate LG will turn off and remain off when the inductor current falls to zero. Diode emulation mode will allow start-up into a pre-biased output voltage. When soft-start is greater than 0.7 V, the LM3150 controller will remain in continuous conduction mode. During diode emulation mode at current limit the low-gate will remain off when the inductor current is off.

The soft-start time should be greater than the input voltage rise time and also satisfy the following equality to maintain a smooth transition of the output voltage to the programmed regulation voltage during start-up.

8.3.7 Thermal Protection

The LM3150 controller should be operated such that the junction temperature does not exceed the maximum operating junction temperature. An internal thermal shutdown circuit, which activates at 165°C (typical), takes the controller to a low-power reset state by disabling the buck switch and the on-timer, and grounding the SS pin. This feature helps prevent catastrophic failures from accidental device overheating. When the junction temperature falls back below 150°C the SS pin is released and device operation resumes.