Micrel, Inc.

MIC5163

April 2009

11

M9999-042209-A

VDDQ

GND

VREF

120pF

A separate voltage source can be used to externally set

the reference point, bypassing the divider network. Also,

provides the drive voltage to enhance the external N-

Channel

MOSFETs.

A

small

1μF

capacitor

is

without exceeding 6V. For example, on an DDR3

0.75V. If the N-Channel MOSFET selected requires a

of 3.25V

Feedback and Compensation

The feedback provides the path for the error amplifier to

amplifier to be correctly externally compensated. For

most applications, a 510Ω resistor is recommended. The

COMP pin on the MIC5163 is the output of the internal

error amplifier. By placing a capacitor and resistor

between the COMP pin and the feedback pin, this

coupled with the feedback resistor, places an external

pole and zero on the error amplifier. With a 510Ω

feedback resistor, a minimum 220pF capacitor is

recommended for a 3.5A peak termination circuit. An

increase in the load will require additional N-Channel

MOSFETs and/or increase in output capacitance may

require feedback and/or compensation capacitor values

to be changed to maintain stability. Feedback resistor

values should not exceed 10kΩ and compensation

capacitors should not be less than 40pF.

Enable

The MIC5163 features an active high enable input. In the

off mode state, leakage currents are reduced to

microamperes. The enable input has thresholds

compatible with TTL/CMOS for simple logic interfacing.

functionality. Do not float the enable pin. Floating this pin

causes the enable to be in an indeterminate state.

Input Capacitance

Although the MIC5163 does not require an input

capacitor for stability, using one greatly improves device

performance. Due to the high-speed nature of the

MIC5163, low ESR capacitors such as Oscon and

ceramics are recommended for bypassing the input. The

recommended value of capacitance will depend greatly

on the proximity to the bulk capacitance. Although a

10μF ceramic capacitor will suffice for most applications,

input capacitance may need to be increased in cases

where the termination circuit is greater than 1-inch away

from the bulk capacitance.

Output Capacitance

Large, low ESR capacitors are recommended for the

capacitors are not required for stability, they are

recommended to reduce the effects of high-speed

the transient condition will be the effect of the peak

current multiplied by the output capacitor’s ESR. For that

reason, Oscon type capacitors and ceramic are excellent

choices for this application. Oscon capacitors have

extremely low ESR and a large capacitance-to-size ratio.

Ceramic capacitors are also well suited to termination

due to their low ESR. These capacitors should have a

dielectric rating of X5R or X7R. Y5V and Z5U type

capacitors are not recommended, due to their poor

performance at high frequencies and over temperature.

The minimum recommended capacitance for a 3.5A

peak circuit is 100μF. Output capacitance can be

increased to achieve greater transient performance.

MOSFET Selection

The MIC5163 utilizes external N-Channel MOSFETs to

sink and source current. MOSFET selection will settle to

MOSFET Power Requirements

One of the most important factors is to determine the

amount of power the MOSFET is going to be required to

dissipate. Power dissipation in a DDR3 circuit will be

identical for both the high side and low side MOSFETs.

both MOSFETs have the same voltage dropped across

them. They are also required to be able to sink and

source the same amount of current (for either all 0s or all

1s). This equates to each side being able to dissipate

the same amount of power. Power dissipation

calculation for the high-side MOSFET is as follows: