MIC25400YML TR Micrel Inc, MIC25400YML TR Datasheet - Page 17

• Manufacturer Part Number: MIC25400YML TR
• Description: IC REG PWM SYNC BUCK 2A 24MLF
• Manufacturer: Micrel Inc
• Series: -r

Specifications of MIC25400YML TR

• Pwm Type: Voltage Mode
• Number Of Outputs: 2
• Frequency - Max: 1.2MHz
• Duty Cycle: 75%
• Voltage - Supply: 4.5 V ~ 13.2 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 24-VFQFN Exposed Pad, 24-MLF®
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant
• Other names: 576-3902-2

The MOSFET is subjected to a V
voltage. A safety factor of 20% should be added to the
V
due to circuit parasitics. Generally, 30V MOSFETs are
recommended for all applications since lower V
MOSFETs tend to have a V
the recommended 4.5V.
RMS Current and MOSFET Power Dissipation
Calculation
Switching loss in the low-side MOSFET can be
neglected since it is turned on and off at a V
The power dissipated in the MOSFET is mostly
conduction loss during the on-time (P
Where:
RDS
The RMS value of the MOSFET current is:
Where:
D is the duty-cycle of the converter
I
Where:
η is the efficiency of the converter.
External Schottky Diode
A freewheeling diode in parallel with the low-side FET is
needed to maintain continuous inductor current flow
while both MOSFETs are turned off (dead-time). Dead-
time is necessary to prevent current from flowing
unimpeded through both MOSFETs. An external
Schottky diode is used to bypass the low-side
MOSFET’s parasitic body diode. An external diode
improves efficiency due to its lower forward voltage drop
as compared to the internal parasitic diode in the FET. It
may also decrease high frequency noise because the
Schottky diode junction does not suffer from reverse
recovery.
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. Depending on the circuit
components and operating conditions, an external
Schottky diode may give up to 1% improvement in
efficiency.
PP
Micrel, Inc.
January 2011
DS(max)
is the inductor ripple current
ON
is the on resistance of the MOSFET switch.
of the MOSFET to account for voltage spikes
P
I
D
SW_RMS
CONDUCTION
=
η
V
⋅
OUT
V
IN
=
(1
=
−
I
SWITCH
D)
⋅
(I
GS
OUT_MAX
RMS
rating that is lower than
2
⋅
DS
RDS
CONDUCTION
2
equal to the input
+
ON
I
PP
12
2
)
).
DS
DS
of 0V.
rated
17
Compensation
The voltage regulation, filter and power stage section is
shown in Figure 12. The error amplifier regulates the
output voltage and compensates the voltage regulation
loop. It is a simplified type III compensator utilizing two
compensating zeros and two poles. Figure 12 also
shows the transfer function for each section.
Compensation is necessary to insure the control loop
has adequate bandwidth and phase margin to properly
respond to input voltage and output current transients.
High gain at DC and low frequencies is needed for
accurate output voltage regulation. Attenuation near the
switching frequency prevents switching frequency noise
from interfering with the control loop.
The output filter contains a complex double pole formed
by the capacitor and inductor and a zero from the output
capacitor and its ESR. The transfer function of the filter
is:
Where:
The Modulator Gain is proportional to the input voltage
and inversely proportional to the internal ramp voltage
generated by the oscillator. The peak-peak ramp voltage
is 1V.
The output voltage divider attenuates V
back to the error amplifier. The divider gain is:
H
Gfilter(s)
ωz
ωo
Q
Gmod
=
=
=
=
R1
R
C
⋅
R4
=
+
C
O
⎛
⎜ ⎜
⎝
R4
=
O
C
⋅
1
L
V
1
R
O
1
⋅
V
RAMP
ESR
L
+
=
IN
O
Q
V
V
OUT
REF
⎞
⎟ ⎟
⎠
s
1
ωo
+
ωz
+
s
ωo
s
2
M9999-020111-C
OUT
and feeds it
MIC25400