MIC2164 MICREL [Micrel Semiconductor], MIC2164 Datasheet - Page 17

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

Micrel, Inc.
Where:
D = duty cycle
C
f
As described in the “Theory of Operation” subsection in
“Functional Description”, MIC2164/-2/-3 requires at least
20mV peak-to-peak ripple at the FB pin to make the gm
amplifier and the error comparator to behavior properly.
Also, the output voltage ripple should be in phase with
the inductor current. Therefore, the output voltage ripple
caused by the output capacitor C
smaller than the ripple caused by the output capacitor
ESR. If low ESR capacitors are selected as the output
capacitors, such as ceramic capacitors, a ripple injection
method is applied to provide the enough FB voltage
ripples. Please refer to the “Ripple Injection” subsection
for more details.
The voltage rating of the capacitor should be twice the
output voltage for a tantalum and 20% greater for
aluminum electrolytic or OS-CON. The output capacitor
RMS current is calculated below:
The power dissipated in the output capacitor is:
Input Capacitor Selection
The input capacitor for the power stage input V
should be selected for ripple current rating and voltage
rating. Tantalum input capacitors may fail when
subjected to high inrush currents, caused by turning the
input supply on. A tantalum input capacitor’s voltage
rating should be at least two times the maximum input
voltage to maximize reliability. Aluminum electrolytic,
OS-CON, and multilayer polymer film capacitors can
handle the higher inrush currents without voltage de-
rating. The input voltage ripple will primarily depend
upon the input capacitor’s ESR. The peak input current
is equal to the peak inductor current, so:
The input capacitor must be rated for the input current
ripple. The RMS value of input capacitor current is
determined at the maximum output current. Assuming
the peak-to-peak inductor current ripple is low:
The power dissipated in the input capacitor is:
Voltage Setting Components
The MIC2164/-2/-3 requires two resistors to set the
SW
September 2009
OUT
= switching frequency
P
I
CIN
= output capacitance value
DISS(CIN)
(
RMS
I
P
Δ
)
C
= I
DISS(C
V
OUT
≈
IN
CIN(RMS)
I
OUT
(RMS)
=
OUT
I
( L
(max)
)
PK
=
2
×ESR
=
ΔI
)
I
C
×
×
L(PP)
12
OUT
ESR
D
CIN
(RMS)
×
CIN
(
1
2
−
OUT
⋅
D
ESR
)
should be much
C
OUT
(20)
(21)
(22)
(23)
(24)
HSD
17
output voltage, as shown in Figure 5.
The output voltage is determined by the equation:
where V
3kΩ and 10kΩ. If R1 is too large, it may allow noise to be
introduced into the voltage feedback loop. If R1 is too
small in value, it will decrease the efficiency of the power
supply, especially at light loads. Once R1 is selected, R2
can be calculated using:
External Schottky Diode (Optional)
An external freewheeling diode, which is 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.
The reverse voltage requirement of the diode is:
The power dissipated by the Schottky diode is:
where, V
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 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
V
I
P
D(avg)
DIODE(rrm)
DIODE
REF
F
Figure 5. Voltage-Divider Configuration
= forward voltage at the peak diode current.
=
=
= 0.8V. A typical value of R1 can be between
R2
V
I
I
OUT
D(avg)
OUT
=
=
V
⋅
HSD
V
2
=
×
OUT
⋅
V
V
30ns
V
REF
REF
F
−
⋅
⋅
V
R1
⋅
(1
REF
f
SW
+
R2
R1
)
M9999-090409-B
MIC2164/-2/-3
(25)
(26)
(27)
(28)