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DETAILED DESCRIPTION

Power Conversion

Control

Enable

Load Disconnect

Soft Start

Output Overvoltage Protection

TPS63700

SLVS530 – SEPTEMBER 2005

The TPS63700 is a dc-dc converter for negative output voltages using buck-boost topology. It operates with an

input voltage range of 2.7 V to 5.5 V and generates a negative output voltage down to –15 V. The output is

controlled by a fixed-frequency, pulse-width-modulated (PWM) regulator. In normal operation mode, the

converter operates at continuous conduction mode (CCM). At light loads it can enter discontinuous conduction

mode (DCM).

The converter operates in a fixed-frequency, pulse-width-modulated 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 inductor

connected to the converter is charged with current. In the remaining time, the time period set by the fixed

operating frequency, the inductor discharges into the output capacitor via the rectifier diode. Usually, at higher

loads the inductor current is continuous. During light load, the inductor current of this converter can become

discontinuous. In this case, the control circuit of the controller output automatically takes care of these changing

conditions to always operate with an optimum control setup.

The controller circuit of the converter is based on a fixed-frequency, multiple-feedforward controller topology.

Input voltage, output voltage, and voltage drop across the switch are monitored and forwarded to the regulator.

Changes in the operating conditions of the converter directly affect the duty cycle.

The error amplifier compares the voltage on FB pin with GND to generate an accurate and stable output voltage.

The error amplifier is internally compensated. At light loads, the converter operates in discontinuous conduction

mode (DCM).

If the load will be further decreased, the energy transmitted to the output capacitor can't be absorbed by the load

and would lead to an increase of the output voltage. In this case, the converter limits the output voltage increase

by skipping switch pulses.

Applying GND signal at the EN pin disables the converter, where all internal circuitry is turned off. The device

now just consumes low shutdown current flowing into the VIN pin. The output load of the converter is also

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

The device supports complete load disconnection when the converter is disabled. The converter turns off the

internal PMOS switch, thus no DC current path remains between load and input voltage source.

The converter has a soft-start function. When the converter is enabled, the implemented switch current limit

ramps up slowly to its nominal value. Soft start is implemented to limit the input current during start-up to avoid

high peak currents at the battery which could interfere with other systems connected to the same battery.

Without soft start, uncontrolled input peak currents flow to charge up the output capacitors and to supply the load

during start-up. This would cause significant voltage drops across the series resistance of the battery and its

connections.

The converter has an implemented output overvoltage protection. The output voltage is limited to –19 V in case

the feedback connection from the output to the FB pin is open.

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