MIC2165_1011 Datasheet, PDF (20/28 Pages) Micrel Semiconductor – Adaptive On-Time DC-DC Controller Featuring HyperLight Load®

Micrel, Inc.
MIC2165
Solving for R_S:
R_S =
L_P
C_p
With the feedforward capacitor, V_FB ripple is very close to
the output voltage ripple:
(33)
ΔV_FB(pp) ≈ ΔV_OUT
(36)
Figure 7 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
Equation 34:
3) Virtually no ripple at V_OUT due to the very low ESR of the
output capacitors.
In this situation, the output voltage ripple is less than 20mV.
Therefore, additional ripple is injected into the FB pin from
the switching node SW via a resistor R_inj and a capacitor
C_inj, as shown in Figure 8c. The injected ripple is:
P_SNUBBER = f_SW × C_S × V_IN2
(34)
Ripple Injection
The V_FB ripple required for proper operation of the
MIC2165 g_m amplifier and error comparator is 20mV to
100mV. However, the output voltage ripple is generally
designed as 1% to 2% of the voltage. For a low output
voltage, such as a 1V output, 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 MIC2165 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.
The applications are divided into three situations according
to the amount of the V_FB ripple:
1) Enough ripple at V_OUT due to the large ESR of the
output capacitors.
As shown in Figure 8a, the converter is stable without any
ripple injection. The V_FB ripple is:
ΔV_FB(PP)
=
V_IN
× K _div
× D × (1- D) ×
1
f_SW × τ
K _div
=
R1//R2
R_inj + R1//R2
(37)
(38)
where:
V_IN = Power stage input voltage at VIN pin
D = Duty Cycle
f_SW = switching frequency
τ = (R1// R2 // R_inj ) ⋅ C_ff
In Equations 37 and 38, it is assumed that the time
constant associated with C_ff must be much greater than
the switching period:
1 ₌ T _{<< 1}
f_SW × τ τ
ΔV_FB(pp)
=
R2
R1+ R2
×
ΔV_OUT
(35)
where: ΔV_OUT = ESR_COUT ⋅ ΔI_L(PP) , ΔI_L(PP) is the peak-to-
peak value of the inductor current ripple.
If the voltage divider resistors R1 and R2 are in the kΩ
range, a C_ff of 1nF to 100nF can easily satisfy the large
time constant consumption. Also, a 100nF injection
capacitor C_inj is used in order to be considered as short for
a wide range of the frequencies.
2) Inadequate ripple at V_OUT due to the small ESR of the
output capacitors.
The output voltage ripple is fed into the FB pin through a
feedforward capacitor C_ff in this situation, as shown in
Figure 8b. The typical C_ff value is between 1nF and 100nF.
Figure 8a.
R2
R1+ R2
× ΔV_OUT
>
20mV
September 2010
20
M9999-092410-E