PWM mode at heavy load

PFM mode at light load

Output Voltage

t

one PFM cycle

Inductor

Current

(loop controlled)

TPS61235, TPS61236

SLVSCK4 – SEPTEMBER 2015

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8.3 Feature Description

8.3.1 Boost Controller Operation

The TPS6123x synchronous boost converter typically operates at a quasi-constant 1-MHz frequency Pulse Width

Modulation (PWM) at moderate to heavy load, which allows the use of small inductors and capacitors to achieve

a small solution size. At light load, it operates in power-save mode with Pulse Frequency Modulation (PFM) for

improved efficiency.

During PWM operation, the converter uses a quasi-constant on-time valley current mode control scheme to

At the beginning of the switching cycle, the low-side NMOS switch is turned on and the inductor current ramps up

to a peak current that is determined by the on-time and the inductance. Once the on-time has expired, the low-

side switch is turned off and the rectifying NMOS switch is turned on. The inductor current decays until reaching

the valley current threshold which is determined by internal control loops. Once this occurs, the on-time is set

again to turn the low-side switch back on and the cycle is repeated. Internal loop compensation is implemented

to simplify the design process while minimizing the number of external components. A bootstrap circuit is built in

to drive the rectifying NMOS switch. Figure 13 illustrates the PWM mode operation.

Figure 13. PWM Mode Operation Illustration

Under a light load condition, the converter works in Pulse Frequency Modulation (PFM) mode. In this mode, the

switching and consumes less quiescent current. It resumes switching when the output voltage drops below the

threshold. The converter exits PFM mode when the output current can no longer be supported in this mode.

Refer to Figure 14 for PFM mode operation details.

Figure 14. PFM Mode Operation Illustration

8.3.2 Soft Start

The TPS6123x integrates an internal soft start circuit which controls ramp up of the output voltage and prevents

the converter from inrush current during start-up.

When the device is enabled, the rectifying switch is turned on to charge the output capacitor to the input voltage.

This is called the pre-charge phase. During the phase, the output current is limited to the pre-charge current limit

gets higher.

10

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