MIC5163_09 Datasheet, PDF (11/15 Pages) Micrel Semiconductor – Dual Regulator Controller for DDR3 GDDR3/4/5 Memory Termination

Micrel, Inc.
VDDQ
VREF
120pF
GND
Figure 5. V_DDQ Divided Down to Provide V_REF
V_REF can also be manipulated for different applications.
A separate voltage source can be used to externally set
the reference point, bypassing the divider network. Also,
external resistors can be added from V_REF-to-ground or
V_REF-to-V_DDQ to shift the reference point up or down.
V_CC
V_CC supplies the internal circuitry of the MIC5163 and
provides the drive voltage to enhance the external N-
Channel MOSFETs. A small 1μF capacitor is
recommended for bypassing the V_CC pin. The minimum
V_CC voltage should be a gate-source voltage above V_TT
without exceeding 6V. For example, on an DDR3
compliant terminator, V_DDQ equals 1.5V and V_TT equals
0.75V. If the N-Channel MOSFET selected requires a
gate source voltage of 2.5V, V_CC should be a minimum
of 3.25V
Feedback and Compensation
The feedback provides the path for the error amplifier to
regulate V_TT. An external resistor must be placed
between the feedback and V_TT. This allows the error
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
MIC5163
compatible with TTL/CMOS for simple logic interfacing.
The enable pin can be tied directly to V_DDQ or V_CC for
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
output (V_TT) of the MIC5163. Although low ESR
capacitors are not required for stability, they are
recommended to reduce the effects of high-speed
current transients on V_TT. The change in voltage during
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
two main categories: size and gate threshold (V_GS).
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.
Since the supply voltage is divided by half to supply V_TT,
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:
April 2009
11
M9999-042209-A