High ECO Threshold

ECO Mode at Light Load

PWM Mode at Heavy Load

Load current increases

Low ECO Threshold

Target Output Voltage

LM3242

SNOSB48E – OCTOBER 2011 – REVISED AUGUST 2015

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7.4.1 PWM Mode Operation

While in PWM mode operation, the converter operates as a voltage-mode controller with input voltage

feedforward. This allows the converter to achieve excellent load and line regulation. The DC gain of the power

stage is proportional to the input voltage. To eliminate this dependence, feed forward inversely proportional to the

input voltage is introduced. While in PWM mode, the output voltage is regulated by switching at a constant

frequency and then modulating the energy per cycle to control power to the load. At the beginning of each clock

cycle the PFET switch is turned on and the inductor current ramps up until the comparator trips and the control

logic turns off the switch. The current limit comparator can also turn off the switch in case the current limit of the

PFET is exceeded. Then the NFET switch is turned on and the inductor current ramps down. The next cycle is

initiated by the clock turning off the NFET and turning on the PFET.

7.4.2 Bypass Mode Operation

The LM3242 contains an internal BPFET switch for bypassing the PWM DC-DC converter during Bypass mode.

In Bypass mode, this BPFET is turned on to power the PA directly from the battery for maximum RF output

power. When the part operates in the Bypass mode, the output voltage is the input voltage less the voltage drop

across the resistance of the BPFET in parallel with the PFET + Switch Inductor. Bypass mode is more efficient

than operating in PWM mode at 100% duty cycle because the combined resistance is significantly less than the

series resistance of the PWM PFET and inductor. This translates into higher voltage available on the output in

Bypass mode, for a given battery voltage. The part can be set to bypass mode by sending BPEN pin high. This

is called Forced Bypass Mode and it remains in bypass mode until BPEN pin goes low. Alternatively the part can

go into Bypass mode automatically. This is called Auto-Bypass mode or Automatic Bypass mode. The bypass

switch turns on when the difference between the input voltage and programmed output voltage is less than 200

mV (typical) for longer than 10 µs (typical). The bypass switch turns off when the input voltage is higher than the

programmed output voltage by 250 mV (typical) for longer than 0.1 µs (typical). This method is very system

resource friendly in that the Bypass PFET is turned on automatically when the input voltage gets close to the

output voltage, a typical scenario of a discharging battery. It is also turned off automatically when the input

7.4.3 ECO Mode Operation

At very light loads (50 mA to 100 mA), the LM3242 enters ECO mode operation with reduced switching

frequency and supply current to maintain high efficiency. During ECO mode operation, the LM3242 positions the

output voltage slightly higher (7 mV typical) than the normal output voltage during PWM mode operation, allowing

additional headroom for voltage drop during a load transient from light to heavy load.

Figure 22. Operation In ECO Mode and Transfer to PWM Mode

7.4.4 Sleep Mode Operation

When VCON is less than 80 mV in 10 µs, the LM3242 goes into SLEEP mode — the SW pin is in Tri-state

(floating), which operates like ECO mode with no switching. The LM3242 device returns to normal operation

immediately when VCON

≥ 130 mV in PWM mode or ECO mode, depending on load detection.

14

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