STOD1412

Introduction

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Introduction

The STOD1412 is a dual DC-DC converter which produces one positive and one negative

output voltage that are each independently regulated and the values of which can be

adjusted with external resistors. Each DC-DC converter is able to supply up to 120 mA of

current with input voltage ranging from 2.7 V and 5.5 V. The device uses a fixed-frequency

PWM controller at 1.3 MHz. This control scheme simplifies noise filtering in sensitive

applications and provides excellent line regulation.

The operation of the STOD1412 can be best understood by referring to the block diagram in

Figure 3, where the step-up control circuit is shown, and a similar scheme is adopted for the

inverting section. At the start of each oscillator cycle, the SR latch is set, which turns on

power switch SW1. A voltage proportional to the switch current is added to the sawtooth

ramp and the resulting sum is fed into the positive terminal of the PWM comparator A2.

When this voltage exceeds the level of the negative input of A2, the SR latch is reset, thus

turning off the power switch. The voltage level of the negative input of A2 is set by the error

amplifier A1, and it is simply an amplified version of the difference between the feedback

voltage and the reference voltage. In this manner, the error amplifier sets the correct peak

current level necessary to keep the output in regulation. If the error amplifier output

increases, more current is delivered to the output; if it decreases, less current is delivered.

The device also has a current limit circuit (not shown in Figure 2). The switch current is

the state of comparator A2. This current limit helps protecting the power switch as well as

the external components connected to the device.

The step up converter works in continuous mode detector (CMD) in the entire line and load

range, while the inverting converter can work in both discontinuous mode detector (DMD)

and CMD.

Figure 3.

PWM control scheme