Micrel, Inc. MIC5162
March 2010 8
M9999-033110
Figure 3. VREF Follows VDDQ
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-V
DDQ
or
V
REF
-to-ground to shift the reference point up or down.
V
CC
V
CC
supplies the internal circuitry of the MIC5162 and
provides the drive voltage to enhance the external N-
Channel MOSFETs. A 1µF ceramic capacitor is
recommended for bypassing the V
CC
pin. The minimum
V
CC
voltage should be a gate-source voltage above V
TT
or greater than 3V without exceeding 6V. 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.
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.
The COMP pin on the MIC5162 is the output of the
internal error amplifier. By placing a capacitor between
the COMP pin and the feedback pin, this coupled with
the feedback resistor places an external pole on the
error amplifier. With a 1kΩ or 510Ω feedback resistor, a
minimum 220pF capacitor is recommended for a 3.5A
peak termination circuit. Increases in load, multiple N-
Channel MOSFETs and/or increase in output
capacitance may require feedback and/or compensation
capacitor values to be increased to maintain stability.
Feedback resistor values should not exceed 10kΩ and
compensation capacitors should not be less than 40pF.
Enable
The MIC5162 features an active-high enable (EN) input.
In the off-mode state, leakage currents are reduced to
microamperes. EN has thresholds compatible with
TTL/CMOS for simple logic interfacing. 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.
Input Capacitance
Although the MIC5162 does not require an input
capacitor for stability, using one greatly improves device
performance. Due to the high-speed nature of the
MIC5162, low ESR capacitors such as OS-CON and
ceramics are recommended for bypassing the input. The
recommended value of capacitance will depend greatly
upon 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 (VTT) of the MIC5162. Although low ESR
capacitors are not required for stability, they are
recommended to reduce the effects of high-speed
current transients on VTT. 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 3 Amp peak circuit is 100µF. Output
capacitance can be increased to achieve greater
transient performance.
MOSFET Selection
The MIC5162 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 required to dissipate.
Power dissipation in an SSTL 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 driver is as follows:
P
D
= (V
DDQ
− V
TT
) × I_SOURCE
where I_SOURCE is the average source current. Power
dissipation for the low-side MOSFET is as follows:
P
D
= V
TT
× I_SINK