Digital Record & Replay Circuit Analysis, Pt. 3

Part Two of this series looks at the DC voltage regulator used in Radio Shack’s digital record and playback module. The TO92 package LDO regulator is perfect for this application because it is simple, efficient and it is an inexpensive component.

Setup For The Measurement

Pictured above Oscium’s iMSO-204 oscilloscope accessory for iOS devices is seen with Radio Shack’s digital record and playback module. The module, part number 276-1323, uses a single IC to record and playback up to 20 seconds of voice or other audio. In this week’s blog post, we look at power distribution within the board and to the external microphone and speaker.

Holtek HT7550-1 LDO Regulator

The voltage regulator in Radio Shack’s digital record and playback module is a +5 volt fixed output, low drop out regulator, Holtek part number HT7550-1. This class of device is frequently used in battery operated applications because of the following features:
  • Low power consumption (Efficient, small quiescent current)
  • Low voltage drop (Accurate as current sourced to the load changes)
  • High input voltage range (Practical for a variety of application)

Pin Assignment Diagram

There are two waveforms measured and plotted in the figure above using iMSO and the iPad display. The regulated 5-volt supply is the blue trace and the green trace represents the peak voltage between the speaker and the module’s ground (reference). It’s interesting to note that at the transition edges of the speaker’s current (green trace), there is a ripple seen in the output of the 5 volt regulator (blue trace). The two figures below show the positive and negative transitions in greater detail where the voltage ripple can be seen.

Ripple or “ringing” at the positive edge of the speaker’s (green) transition between OFF and ON.

Ripple in the negative edge of the transition; less pronounced than the positive going transition.

Ripple At The Transitions

Why is there ripple in the 5V and Speaker voltages? In the module’s Record Mode, current flows from the regulator through the IC and the microphone and is relatively small compared to Playback Mode. When the module is in Playback Mode, current from the regulator flows through the IC and the speaker. The ripple could be caused by insufficient charge storage capacitance near the terminals of the speaker. Alternately, the ripple might be an artifact of ground bounce. When the speaker turns on hard and there’s a sudden current surge from the regulator, it may cause pronounced noise in the ground at switching/transitioning points.

This figure shows the current path for the DC power of the Radio Shack record and replay module. The +9 volt and +5 volt supplies are isolated from each other by the Power Supply Rejection Ratio (PSRR) of the regulator, so a noisy 9V has little or no effect on the +5V. However, both supplies share a common ground which is to be expected, but might also contribute propagated noise across the system.

What Comes Next

In the next blog post we look at some of the microphone circuit. Understanding how the microphone works allows the circuit to modified for use in other applications or tweaked to optimize performance.

  • Part 1: Record & Replay Circuit Analysis
  • Part 2: LDO Regulator Circuit Analysis
  • Part 3: Microphone Circuit Analysis
  • Part 4: Speaker Circuit Analysis