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Power Conversion

Control

Power-Save Mode

Enable

Load Disconnect

TPS65130, TPS65131

SLVS493B – MARCH 2004 – REVISED SEPTEMBER 2004

DETAILED DESCRIPTION (continued)

Both converters operate in a fixed-frequency, PWM control scheme. So, the on-time of the switches varies

depending on input-to-output voltage ratio and the load. During this on-time, the inductors connected to the

converters are charged with current. In the remaining time, the time period set by the fixed operating frequency,

the inductors discharge into the output capacitors via the rectifier diodes. Usually at higher loads, the inductor

currents are continuous. At lighter loads, the boost converter uses an additional internal switch to allow current

flowing back to the input. This avoids inductor current becoming discontinuous in the boost converter. So, the

boost converter is always controlled in a continuous current mode. At the inverting converter, during light loads,

the inductor current can become discontinuous. In this case, the control circuit of the inverting controller output

automatically takes care of these changing conditions to always operate with an optimum control setup.

The controller circuits of both converters are based on a fixed-frequency, multiple-feedforward controller

topology. Input voltage, output voltage, and voltage drop across the switches are monitored and forwarded to the

regulator. Changes in the operating conditions of the converters directly affect the duty cycle and must not take

the indirect and slow way through the output voltage control loops. Measurement errors in this feedforward

system are corrected by a self-learning control system. To avoid output voltage steps due to output changes of

this self-learning control system, its output is dampened by an external capacitor.

The voltage loops, determined by the error amplifiers, only have to handle small signal errors. The error

amplifiers are internally compensated. Their inputs are the feedback voltages on the FBP and FBN pins. These

voltages are compared with the internal reference voltage to generate an accurate and stable output voltage.

The PSN and PSP can be used to select different operating modes. To enable power-save mode for the

corresponding converter, the dedicated PS pin must be set high. Power-save mode can be used to improve

efficiency at light load. In power-save mode, the converter only operates when the output voltage falls below a

set threshold voltage. It ramps up the output voltage with one or several operating pulses and goes again into

power-save mode once the inductor current goes discontinuous. The power-save mode can be disabled

seperately for each converter by setting the corresponding PS pin low.

Applying a low signal at the enable ENP or ENN pins shuts down the corresponding converter. When both

enable pins are tied low, the device enters shutdown mode, where all internal circuitry is turned off. The device

now just consumes low shutdown current flowing into the VIN pin. The output loads of the converters are

disconnected from the battery as described in the following paragraph. Pulling the enable pins high enables the

corresponding converter. Internal circuitry, necessary to operate the specific converter, is then turned on.

The device supports completely disconnecting the load, when the converters are disabled. At the inverting

converter, this is done by just turning off the internal PMOS switch. If the inverting converter is turned off, no DC

current path remains which could discharge the battery. This is different at the boost converter. The external

rectifying diode, together with the boost inductor, form a DC current path which could discharge the battery if any

load is connected at the output. The device has no internal switch to prevent current from flowing. For this

reason, a PMOS gate control output (BSW) is implemented. A PMOS switch can be placed into this DC current

path, ideally, directly between the boost inductor and battery. To be able to really disconnect the battery, the

forward direction of the parasitic backgate diode of this switch must point to the battery. The external PMOS

switch, connected to BSW, turns on when the boost converter is enabled and is turned off when the boost

converter is disabled.

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