MIC5165 Datasheet, PDF (12/24 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
VREF 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-V_DDQ or
V_REF-to-ground to shift the reference point up or down.
VCC
The VCC voltage range is from 3V to 6V, but the
minimum voltage on the VCC pin should consider the
Gate-to-Source voltage of the MOSFET and V_TT voltage.
For example, on a 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.
V_CCmin=V_TT+V_GS
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 MIC5165 is the output of the
internal error amplifier. By placing a capacitor and
resistor between the COMP pin and the FB 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.
Enable
EN can be tied directly to V_DDQ or V_CC for functionality.
Do not float the EN pin. Floating this pin causes the
enable circuitry to be in an indeterminate state.
Power Good
Power Good signal output remains high as long as
output is within ±10% range of regulated V_TT and goes
MIC5165
low if output moves beyond this range.
Input Capacitance
The MIC5165 application operates in the linear region
during the steady state condition, so there are no
switching current pulses from the input capacitor. The
application does not require a bulk input capacitor, but
using one greatly improves device performance during
fast load transients. Since output voltage V_TT follows the
input voltage V_DDQ attention should be given to possible
voltage dips on VDDQ pin. Capacitors with low ESR
such as OS-CON and ceramics are recommended for
bypassing the input. Although a 100μF 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 MIC5165. 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, OS-CON type capacitors and ceramic are
excellent choices for this application. OS-CON
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 MIC5165 utilizes external N-Channel MOSFETs to
sink and source current. MOSFET selection will be
determined by two main characteristics: size and gate
threshold (V_GS).
MOSFET Power Requirements
One of the most important factors to determine is 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
June 2010
12
M9999-061510-B