MIC2164 MICREL [Micrel Semiconductor], MIC2164 Datasheet - Page 18

• Manufacturer Part Number: MIC2164
• Description: Constant Frequency, Synchronous Buck Controllers Featuring Adaptive On-Time Control
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
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 upon 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
MIC2164/-2/-3 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 MIC2164/-2/-3 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:
where ΔI
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:
3) Invisible ripple at the FB voltage due to the very low
ESR of the output capacitors.
September 2009
ΔV
FB(pp)
L(pp)
ΔV
=
is the peak-to-peak value of the inductor
R1
FB(pp)
R2
+
R2
≈
ESR
⋅
ESR
⋅
C
ΔI
OUT
L
(pp)
⋅
ΔI
L
(pp)
(29)
(30)
18
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:
where
V
D = Duty Cycle
f
In the formula (31) and (32), it is assumed that the time
constant associated with Cff must be much greater than
the switching period:
τ
SW
HSD
=
= switching frequency
R (
ΔV
= Power stage input voltage at HSD pin
1
//
FB(pp)
R
2
Figure 6b. Inadequate Ripple at FB
//
K
=
Figure 6c. Invisible Ripple at FB
Figure 6a. Enough Ripple at FB
Rinj
div
V
HSD
=
)
⋅
×
Rinj
Cff
K
div
R1//R2
+
×
R1//R2
D
×
(1
-
D)
×
f
SW
M9999-090409-B
1
×
MIC2164/-2/-3
τ
(31)
(32)