MIC2166_1009 Datasheet, PDF (18/28 Pages) Micrel Semiconductor – Adaptive On-Time DC-DC Controller Hyper Speed Control™ Family

Micrel, Inc.
The snubber components should be placed as close as
possible to the low-side MOSFET and/or external
Schottky diode since it contributes to most of the stray
capacitance. Placing the snubber too far from the
MOSFET or using a trace that is too long or thin will add
inductance to the snubber and diminishes its
effectiveness.
A proper snubber design requires that the parasitic
inductance and capacitance be known. A method of
determining these values and calculating the damping
resistor value is outlined below:
1. Measure the ringing frequency at the switch node
which is determined by parasitic L_P and C_P. Define
this frequency as f₁.
2. Add a capacitor C_S (such as 2 times as big as the
C_OSS of the FET) from the switch node to ground and
measure the new ringing frequency. Define this new
(lower) frequency as f₂. L_P and C_P can now be
solved using the values of f₁, f₂ and C_S.
3. Add a resistor R_S in series with C_S to generate
critical damping.
Step 1: First measure the ringing frequency on the
switch node voltage when the high-side MOSFET turns
on. This ringing is characterized by the equation:
f₁ = _2π
1
L_P × C_P
(27)
where C_P and L_P are the parasitic capacitance and
inductance.
Step 2: Add a capacitor, C_S, in parallel with the
synchronous MOSFET, Q2. The capacitor value should
be approximately 2 times the C_OSS of Q2. Measure the
frequency of the switch node ringing, f₂:
f₂
=
2π
1
Lp × (Cs + Cp)
(28)
Define f’ as:
f ^' = ^f1
f₂
Combining the equations for f₁, f₂ and f’ to derive C_P, the
parasitic capacitance:
C_P
=
C_S
(f^' )² −
1
MIC2166
(29)
L_P is solved by re-arranging the equation for f₁:
L_P
=
(2π)²
1
× C_P
× (f₁)²
(30)
Step 3: Calculate the damping resistor.
Critical damping occurs at Q = 1:
Q = R_S ×
C_{P = 1}
L_P
(31)
Solving for R_S:
R_S =
L_P
C_p
(32)
Figure 6 shows the snubber in the circuit and the
damped switch node waveform. The snubber capacitor,
C_S, is charged and discharged each switching cycle. The
energy stored in C_S is dissipated by the snubber resistor,
R_S, two times per switching period. This power is
calculated in the equation below:
P_SNUBBER = f_SW × C_S × V_IN2
(33)
Ripple Injection
The V_FB ripple required for proper operation of the
MIC2166 g_m amplifier and error comparator is 20mV to
100mV. However, the output voltage ripple is generally
designed as 1% to 2% of the output voltage. For a low
output voltage, such as a 1V, the output voltage ripple is
only 10mV to 20mV, and the V_FB ripple is less than
20mV. If the V_FB ripple is so small that the g_m amplifier
and error comparator cannot sense it, the MIC2166 will
lose control and the output voltage is not regulated. In
order to have some amount of V_FB ripple, a ripple
injection method is applied for low output voltage ripple
applications.
September 2010
18
M9999-092410-C