MIC5164 Datasheet, PDF (10/23 Pages) Micrel Semiconductor – Dual Regulator Controller for DDR3 GDDR3/4/5 Memory and High-Speed Bus Termination

Micrel, Inc.
MIC5164
Figure 4. V_REF Follows V_DDQ
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 an SSTL compliant terminator, V_DDQ
equals 2.5V and V_TT equals 1.25V. If the N-Channel
MOSFET selected requires a gate source voltage of
2.5V, V_CC should be a minimum of 3.75V.
V_CCmin=V_TT+V_GS
Feedback and Compensation
The feedback (FB) pin is connected to V_TT for regulation.
An external resistor must be placed between FB 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 MIC5164 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 FB
resistor, a minimum 220pF capacitor is recommended
for a 3A 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 undetermined state.
Power Good
PG signal output remains high as long as output is within
±10% range of regulated V_TT and goes low if output
moves beyond this range.
Input Capacitance
The MIC5164 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 MIC5164. 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 are excellent for this
application. They have extremely low ESR and 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 3A peak circuit is 100μF. Output
capacitance can be increased to achieve greater
transient performance.
MOSFET Selection
The MIC5164 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 required to dissipate.
June 2010
10
M9999-061510