LM3200

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SNVS319C – NOVEMBER 2004 – REVISED APRIL 2013

The output voltage is regulated by modulating the PFET switch on time to control the average current sent to the

load. The effect is identical to sending a duty-cycle modulated rectangular wave formed by the switch and

synchronous rectifier at SW to a low-pass filter formed by the inductor and output filter capacitor. The output

voltage is equal to the average voltage at the SW pin.

PWM Mode

While in PWM (Pulse Width Modulation) 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. Energy per cycle is set by

modulating the PFET switch on-time pulse width to control the peak inductor current. This is done by comparing

the signal from the PFET drain current to a slope-compensated reference current generated by the error

amplifier. At the beginning of each cycle, the clock turns on the PFET switch, causing the inductor current to

ramp up. When the current sense signal ramps past the error amplifier signal, the PWM comparator turns off the

PFET switch and turns on the NFET synchronous rectifier, ending the first part of the cycle. If an increase in load

pulls the output down, the error amplifier output increases, which allows the inductor current to ramp higher

before the comparator turns off the PFET. This increases the average current sent to the output and adjusts for

the increase in the load. Before appearing at the PWM comparator, a slope compensation ramp from the

oscillator is subtracted from the error signal for stability of the current feedback loop. The minimum on-time of

PFET in PWM mode is 50 ns (typ).

Bypass Mode

The LM3200 contains an internal PFET switch for bypassing the PWM DC-DC converter during Bypass mode. In

Bypass mode, this PFET 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 will be the input voltage less the voltage drop

across the resistance of the bypass PFET. Bypass mode is more efficient than operating in PWM mode at 100%

duty cycle because the resistance of the bypass PFET is 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 placed in bypass mode by sending BYP pin high. This is called Forced Bypass Mode and it

remains in bypass mode until BYP 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 250 mV (typ.) for more than the bypass delay time of 15 µs (typ.). The bypass switch

turns off when the input voltage is higher than the programmed output voltage by 450 mV (typ.) for longer than

the bypass delay time. The bypass delay time is provided to prevent false triggering into Automatic Bypass mode

automatically when the input voltage gets close to the output voltage, typical scenario of a discharging battery. It

is also turned off automatically when the input voltage rises, typical scenario of a charger connected. Another

recommended to connect BYPOUT pin directly to the output capacitor with a separate trace and not to the FB

pin.

Operating Mode Selection Control

The BYP digital input pin is used to select between PWM/Auto-bypass and Bypass operating mode. Setting BYP

pin high (>1.2V) places the device in Forced Bypass mode. Setting BYP pin low (<0.4V) or leaving it floating

places the device in PWM/Auto-bypass mode.

Bypass and PWM operation overlap during the transition between the two modes. This transition time is

approximately 31 µs when changing from PWM to Bypass mode, and 15 µs when changing from Bypass to

PWM mode. This helps prevent under or overshoots during the transition period between PWM and Bypass

modes.

Shutdown Mode

Setting the EN digital pin low (<0.4V) places the LM3200 in a 0.1 µA (typ.) Shutdown mode. During shutdown,

the PFET switch, NFET synchronous rectifier, reference voltage source, control and bias circuitry of the LM3200

are turned off. Setting EN high (>1.2V) enables normal operation.

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Product Folder Links: LM3200