8.3.1 Fixed Frequency Peak Current Mode Control

The following operation description of the LMR14020 will refer to the Function Block Diagram and to the waveforms in Figure 13. LMR14020 output voltage is regulated by turning on the high-side N-MOSFET with controlled ON time. During high-side switch ON time, the SW pin voltage swings up to approximately V_IN, and the inductor current i_L increase with linear slope (V_IN – V_OUT) / L. When high-side switch is off, inductor current discharges through freewheel diode with a slope of –V_OUT / L. The control parameter of Buck converter is defined as Duty Cycle D = t_ON /T_SW, where t_ON is the high-side switch ON time and T_SW is the switching period. The regulator control loop maintains a constant output voltage by adjusting the duty cycle D. In an ideal Buck converter, where losses are ignored, D is proportional to the output voltage and inversely proportional to the input voltage: D = V_OUT / V_IN.

Figure 13. SW Node and Inductor Current Waveforms in
Continuous Conduction Mode (CCM)

The LMR14020 employs fixed frequency peak current mode control. A voltage feedback loop is used to get accurate DC voltage regulation by adjusting the peak current command based on voltage offset. The peak inductor current is sensed from the high-side switch and compared to the peak current to control the ON time of the high-side switch. The voltage feedback loop is internally compensated, which allows for fewer external components, makes it easy to design, and provides stable operation with almost any combination of output capacitors. The regulator operates with fixed switching frequency at normal load condition. At very light load, the LMR14020 will operate in Sleep-mode to maintain high efficiency and the switching frequency will decrease with reduced load current.

8.3.2 Slope Compensation

The LMR14020 adds a compensating ramp to the MOSFET switch current sense signal. This slope compensation prevents sub-harmonic oscillations at duty cycle greater than 50%. The peak current limit of the high-side switch is not affected by the slope compensation and remains constant over the full duty cycle range.

8.3.3 Sleep-mode

The LMR14020 operates in Sleep-mode at light load currents to improve efficiency by reducing switching and gate drive losses. If the output voltage is within regulation and the peak switch current at the end of any switching cycle is below the current threshold of 300 mA, the device enters Sleep-mode. The Sleep-mode current threshold is the peak switch current level corresponding to a nominal internal COMP voltage of 400 mV.

When in Sleep-mode, the internal COMP voltage is clamped at 400 mV and the high-side MOSFET is inhibited, and the device draws only 40 μA (typical) input quiescent current. Since the device is not switching, the output voltage begins to decay. The voltage control loop responds to the falling output voltage by increasing the internal COMP voltage. The high-side MOSFET is enabled and switching resumes when the error amplifier lifts internal COMP voltage above 400 mV. The output voltage recovers to the regulated value, and internal COMP voltage eventually falls below the Sleep-mode threshold at which time the device again enters Sleep-mode.

8.3.4 Low Dropout Operation and Bootstrap Voltage (BOOT)

The LMR14020 provides an integrated bootstrap voltage regulator. A small capacitor between the BOOT and SW pins provides the gate drive voltage for the high-side MOSFET. The BOOT capacitor is refreshed when the high-side MOSFET is off and the external low side diode conducts. The recommended value of the BOOT capacitor is 0.1 μF. A ceramic capacitor with an X7R or X5R grade dielectric with a voltage rating of 16 V or higher is recommended for stable performance over temperature and voltage.

When operating with a low voltage difference from input to output, the high-side MOSFET of the LMR14020 will operate at approximate 97% duty cycle. When the high-side MOSFET is continuously on for 5 or 6 switching cycles (5 or 6 switching cycles for frequency lower than 1 MHz, and 10 or 11 switching cycles for frequency higher than 1MHz) and the voltage from BOOT to SW drops below 3.2 V, the high-side MOSFET is turned off and an integrated low side MOSFET pulls SW low to recharge the BOOT capacitor.

Since the gate drive current sourced from the BOOT capacitor is small, the high-side MOSFET can remain on for many switching cycles before the MOSFET is turned off to refresh the capacitor. Thus the effective duty cycle of the switching regulator can be high, approaching 97%. The effective duty cycle of the converter during dropout is mainly influenced by the voltage drops across the power MOSFET, the inductor resistance, the low side diode voltage and the printed circuit board resistance.

8.3.5 Adjustable Output Voltage

The internal voltage reference produces a precise 0.75 V (typical) voltage reference over the operating temperature. The output voltage is set by a resistor divider from output voltage to the FB pin. It is recommended to use 1% tolerance or better and temperature coefficient of 100 ppm or lower divider resistors. Select the low side resistor R_FBB for the desired divider current and use Equation 1 to calculate high-side R_FBT. Larger value divider resistors are good for efficiency at light load. However, if the values are too high, the regulator will be more susceptible to noise and voltage errors from the FB input current may become noticeable. R_FBB in the range from 10 kΩ to 100 kΩ is recommended for most applications.

Figure 14. Output Voltage Setting

Equation 1.

8.3.6 Enable and Adjustable Under-voltage Lockout

The LMR14020 is enabled when the VIN pin voltage rises above 3.7 V (typical) and the EN pin voltage exceeds the enable threshold of 1.2 V (typical). The LMR14020 is disabled when the VIN pin voltage falls below 3.52 V (typical) or when the EN pin voltage is below 1.2 V. The EN pin has an internal pull-up current source (typically I_EN = 1 μA) that enables operation of the LMR14020 when the EN pin is floating.

Many applications will benefit from the employment of an enable divider R_ENT and R_ENB in Figure 15 to establish a precision system UVLO level for the stage. System UVLO can be used for supplies operating from utility power as well as battery power. It can be used for sequencing, ensuring reliable operation, or supply protection, such as a battery. An external logic signal can also be used to drive EN input for system sequencing and protection.

When EN terminal voltage exceeds 1.2 V, an additional hysteresis current (typically I_HYS = 3.6 μA) is sourced out of EN terminal. When the EN terminal is pulled below 1.2 V, I_HYS current is removed. This additional current facilitates adjustable input voltage UVLO hysteresis. Use Equation 2 and Equation 3 to calculate R_ENT and R_ENB for desired UVLO hysteresis voltage.

Figure 15. System UVLO By Enable Dividers

Equation 2.

Equation 3.

where V_START is the desired voltage threshold to enable LMR14020, V_STOP is the desired voltage threshold to disable device.

8.3.7 External Soft-start

The LMR14020 has soft-start pin for programmable output ramp up time. The soft-start feature is used to prevent inrush current impacting the LMR14020 and its load when power is first applied. The soft-start time can be programed by connecting an external capacitor C_SS from SS pin to GND. An internal current source (typically I_SS = 3 μA) charges C_SS and generates a ramp from 0 V to V_REF. The soft-start time can be calculated by Equation 4:

Equation 4.

The internal soft-start resets while device is disabled or in thermal shutdown.

8.3.8 Switching Frequency and Synchronization (RT/SYNC)

The switching frequency of the LMR14020 can be programmed by the resistor RT from the RT/SYNC pin and GND pin. The RT/SYNC pin can’t be left floating or shorted to ground. To determine the timing resistance for a given switching frequency, use Equation 5 or the curve in Figure 16. Table 1 gives typical R_T values for a given f_SW.

Equation 5.

Figure 16. RT vs Frequency Curve

The LMR14020 switching action can also be synchronized to an external clock from 250 kHz to 2.3 MHz. Connect a square wave to the RT/SYNC pin through either circuit network shown in Figure 17. Internal oscillator is synchronized by the falling edge of external clock. The recommendations for the external clock include: high level no lower than 1.7 V, low level no higher than 0.5 V and have a pulse width greater than 30 ns. When using a low impedance signal source, the frequency setting resistor R_T is connected in parallel with an AC coupling capacitor C_COUP to a termination resistor R_TERM (e.g., 50 Ω). The two resistors in series provide the default frequency setting resistance when the signal source is turned off. A 10 pF ceramic capacitor can be used for C_COUP. Figure 18, Figure 19 and Figure 20 show the device synchronized to an external system clock.

Figure 17. Synchronizing to an External Clock

Figure 18. Synchronizing in CCM

Figure 20. Synchronizing in Sleep-mode Mode

Figure 19. Synchronizing in DCM

Equation 6 calculates the maximum switching frequency limitation set by the minimum controllable on time and the input to output step down ratio. Setting the switching frequency above this value will cause the regulator to skip switching pulses to achieve the low duty cycle required at maximum input voltage.

Equation 6.

where

• I_OUT = Output current
• R_IND = Inductor series resistance
• V_{IN_MAX} = Maximum input voltage
• V_OUT = Output voltage
• V_D = Diode voltage drop
• R_{DS_ON} = High-side MOSFET switch on resistance
• t_ON = Minimum on time

8.3.9 Over Current and Short Circuit Protection

The LMR14020 is protected from over current condition by cycle-by-cycle current limiting on the peak current of the high-side MOSFET. High-side MOSFET over-current protection is implemented by the nature of the Peak Current Mode control. The high-side switch current is compared to the output of the Error Amplifier (EA) minus slope compensation every switching cycle. Please refer to Functional Block Diagram for more details. The peak current of high-side switch is limited by a clamped maximum peak current threshold which is constant. So the peak current limit of the high-side switch is not affected by the slope compensation and remains constant over the full duty cycle range.

The LMR14020 also implements a frequency fold-back to protect the converter in severe over-current or short conditions. The oscillator frequency is divided by 2, 4, and 8 as the FB pin voltage decrease to 75%, 50%, 25% of V_REF. The frequency fold-back increases the off time by increasing the period of the switching cycle, so that it provides more time for the inductor current to ramp down and leads to a lower average inductor current. Lower frequency also means lower switching loss. Frequency fold-back reduces power dissipation and prevents overheating and potential damage to the device.

8.3.10 Overvoltage Protection

The LMR14020 employs an output overvoltage protection (OVP) circuit to minimize voltage overshoot when recovering from output fault conditions or strong unload transients in designs with low output capacitance. The OVP feature minimizes output overshoot by turning off high-side switch immediately when FB voltage reaches to the rising OVP threshold which is nominally 109% of the internal voltage reference V_REF. When the FB voltage drops below the falling OVP threshold which is nominally 107% of V_REF, the high-side MOSFET resumes normal operation.

8.3.11 Thermal Shutdown

The LMR14020 provides an internal thermal shutdown to protect the device when the junction temperature exceeds 170°C (typical). The high-side MOSFET stops switching when thermal shundown activates. Once the die temperature falls below 158°C (typical), the device reinitiates the power up sequence controlled by the internal soft-start circuitry.