www.latticesemi.com 1 an6077_1.1

October 2014 Application Note AN6077

or product names are trademarks or registered trademarks of their respective holders. The specifications and information herein are subject to change without notice.

Introduction

The Power/Platform Management Device TrimCell DAC outputs are designed to interface with DC power supply cir-

cuits in order to fine-tune their output voltages. In most cases, this adjustment is made by gently influencing the

feedback node of the DC power supply control circuit. This application note addresses typical concerns regarding

DC power supply stability when connected to a Power/Platform Management Device TrimCell DAC output. Some

typical concerns that arise when adding circuitry to the feedback node include the following: control-loop stability,

output noise or ripple, and the settling time. This application note shows that the control-loop stability is not affected

by the addition of the Power/Platform Management device trim circuitry.

Table 1. Applicable Power/Platform Management Device Summary

DC-DC Converter Control Theory

DC-DC power supplies are available in a wide range of voltage, amperage, power, and physical sizes. Regardless

of the specific supply, all are based on the same type of power and control circuitry. Some form of pulse width mod-

ulation (PWM) is used to drive a switcher and filter circuit. The switcher filter circuitry usually consists of a power

switch, an inductor, rectification, and filtering. The output voltage from the switcher converter circuitry is fed back

around to control the PWM circuitry. The feedback loop includes a reference, error amplifier, and feedback compen-

sation as shown in Figure 1. One of the design objectives of the control loop is to have ample phase and gain mar-

gins to provide stable operation over a wide range of conditions. Proper design of the control loop not only affects

the stability but also transient response time and the noise level (the AC component of the DC output).

Device VMONs

Digital

Inputs HVOUTs

Open Drain

Outputs

Trim DAC

Outputs Digital I/O

CPLD

Macrocells

FPGA

LUTs

ispPAC-POWR6AT6 6 — — — 6 — — —

ispPAC-POWR1220AT8 12 6 4 16 8 — 48 —

LPTM10-1247 12 4 4 12 6 31 48 640

LPTM10-12107 12 4 4 12 8 91 48 640

LPTM21 10 — 4 10

495

— 1280

L-ASC10 10 — 4 9

4———

1. Platform Manager 2 Open Drain Outputs can be configured as inputs.

2. Platform Manager 2 Digital I/O count does not include SDA_M, SCL_M, or JTAGENB pins.

Stable Operation of DC-DC Converters

with Power/Platform Management

Devices Closed Loop Trim

Stable Operation of DC-DC Converters

with Power/Platform Management Devices Closed Loop Trim

Figure 1. Simplified Block Diagram of DC-DC Converter Circuit

Power/Platform Management Device TrimCell Function

The Power/Platform Management device closed loop trim feature is needed in systems utilizing inexpensive DC-

DC converters to improve the accuracy of their output voltage. The ability of these devices to trim several different

supplies at once also decreases the cost but increases the accuracy of the entire system.

In Figure 2, a block diagram of the device TrimCell is shown with the block diagram of a generic DC-DC converter.

The output of the DC-DC converter is monitored by the device VMON input and compared with the setpoint in the

TrimCell; the difference between the two, results in an error signal. This error signal is filtered and injected into the

summing node of the DC-DC converter using an interface circuit (shown using R1-R5). The actual interface circuit

between the TrimCell output and the DC-DC converter trim pin depends on the actual converter and the range of

adjustment and generally uses only one to three resistors. The Power/Platform Management design tool (Platform

Designer or PAC-Designer, depending on device generation) calculates the resistor value and displays the inter-

face configuration for a given DC-DC converter and the desired range of trim. The TrimCell DAC output current is

limited to only +/– 200 µA to prevent overdriving DC-DC trim pins. Furthermore, the programmable low-pass filter

that is implemented within the TrimCell is significantly slower than the filter of the DC-DC converter. The TrimCell fil-

ter consists of a programmable DAC update delay that will be discussed in more detail in the next section. In the

frequency domain the two feedback loops are in parallel and not in series. Thus, the DC-DC loop can still respond

to transients in load or supply with little or no change to its specified performance. Meanwhile, the slower TrimCell

loop adjusts for static offsets such as the DC-DC reference value, voltage drop from PC-board traces and/or con-

nectors.

PWM Control

DC-IN

Switcher

and Filter

DC-OUT

Reference, Error Amp, and

Feedback Compensation