MIC2164_10 Datasheet, PDF (19/39 Pages) Micrel Semiconductor – Synchronous Buck Controllers Featuring Adaptive On-Time Control 28V Input, Constant Frequency

Micrel, Inc.
External Schottky Diode (Optional)
An external freewheeling diode, which is generally not
necessary, can be used to keep the inductor current flow
continuous while both MOSFETs are turned off. This
dead time prevents current from flowing unimpeded
through both MOSFETs and is typically 30ns. The diode
conducts twice during each switching cycle. Although the
average current through this diode is small, the diode
must be able to handle the peak current.
I_D(avg) = I_OUT ⋅ 2 ⋅ 30ns ⋅ f_SW
(25)
The reverse voltage requirement of the diode is:
V_DIODE(rrm) = V_HSD
The power dissipated by the Schottky diode is:
P_DIODE = I_D(avg) × V_F
(26)
where, VF = forward voltage at the peak diode current.
The external Schottky diode is not necessary for the
circuit operation since the low-side MOSFET contains a
parasitic body diode. The external diode will improve
efficiency and decrease the high frequency noise. If the
MOSFET body diode is then used, it must be rated to
handle the peak and average current. The body diode
has a relatively slow reverse recovery time and a
relatively high forward voltage drop. The power lost in
the diode is 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 upon the circuit
components and operating conditions, an external
Schottky diode will give a ½ to 1% improvement in
efficiency.
MIC2164/-2/-3/C
Ripple Injection
The minimum FB voltage ripple requested by the
MIC2164/-2/-3 g_m amplifier and error comparator is
20mV (100mV maximum). 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 g_m amplifier and error comparator
could not sense it, then the MIC2164/-2/-3 will lose
control and the output voltage will not be 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 5a, 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)}
(27)
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 5b. 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)}
(28)
3) Invisible ripple at the FB voltage is due to the very low
ESR of the output capacitors.
September 2010
19
M9999-091310-D