MIC2174 Datasheet, PDF (16/24 Pages) Micrel Semiconductor – 300kHz, Synchronous Buck Controller 300kHz, Synchronous Buck Controller

Micrel, Inc.
proportional to the forward voltage drop of the diode. As
the high-side MOSFET starts to turn on, the body diode
becomes a short circuit for the reverse recovery period,
dissipating additional power. The diode recovery and the
circuit inductance will cause ringing during the high-side
MOSFET turn-on.
An external Schottky diode conducts at a lower forward
voltage preventing the body diode in the MOSFET from
turning on. The lower forward voltage drop dissipates
less power than the body diode. The lack of a reverse
recovery mechanism in a Schottky diode causes less
ringing and less power loss. Depending on the circuit
components and operating conditions, an external
Schottky diode will give a 1/2% to 1% improvement in
efficiency.
Ripple Injection
The minimum FB voltage ripple requested by the
MIC2174 gm amplifier and error comparator is 20mV.
However, the output voltage ripple is generally designed
as 1% to 2% of the output voltage. For a low output
voltage, such as 1V output, the output voltage ripple is
only 10mV to 20mV, and the FB voltage ripple is less
than 20mV. If the FB voltage ripple is so small that the
gm amplifier and error comparator could not sense it, the
MIC2174 will lose control and the output voltage is not
regulated. In order to have some amount of FB voltage
ripple, the ripple injection method is applied for low
output voltage ripple applications.
The applications are divided into three situations
according to the amount of the FB voltage ripple:
1) Enough ripple at the FB voltage due to the large ESR
of the output capacitors.
As shown in Figure 6a, the converter is stable without
any adding in this situation. The FB voltage ripple is:
ΔV_FB(pp)
=
R2
R1+ R2
×
ESR
C_OUT
× ΔI_{L (pp)}
(30)
where ΔI_L(pp) is the peak-to-peak value of the inductor
current ripple.
2) Inadequate ripple at the FB voltage due to the small
ESR of the output capacitors.
The output voltage ripple is fed into the FB pin through a
feedforward capacitor Cff in this situation, as shown in
Figure 6b. The typical Cff value is between 1nF to 100nF.
With the feedforward capacitor, the FB voltage ripple is
very close to the output voltage ripple:
ΔV_FB(pp) ≈ ESR × ΔI_L(pp)
(31)
3) Invisible ripple at the FB voltage due to the very low
ESR of the output capacitors.
MIC2174
Figure 6a. Enough Ripple at FB
Figure 6b. Inadequate Ripple at FB
Figure 6c. Invisible Ripple at FB
In this situation, the output voltage ripple is less than
20mV. Therefore, additional ripple is injected into the FB
pin from the switching node LX via a resistor Rinj and a
capacitor Cinj, as shown in Figure 6c. The injected ripple
is:
1
ΔV_FB(pp) = V_HSD ×K _div ×D×(1- D)× _{fSW × τ}
(32)
K _div
=
R1//R2
Rinj + R1//R2
(33)
where
V_HSD = Power stage input voltage at HSD pin
D = Duty Cycle
f_SW = switching frequency
τ = (R1//R2//Rinj) × Cff
In the formula (32) and (33), it is assumed that the time
constant associated with Cff must be much greater than
the switching period:
September 2009
16
M9999-090409-B