MIC2169BYMM Micrel Inc, MIC2169BYMM Datasheet - Page 11

• Manufacturer Part Number: MIC2169BYMM
• Description: IC CTLR PWM BUCK SYNC 10MSOP
• Manufacturer: Micrel Inc

Specifications of MIC2169BYMM

• Pwm Type: Voltage Mode
• Number Of Outputs: 1
• Frequency - Max: 550kHz
• Duty Cycle: 92%
• Voltage - Supply: 3 V ~ 14.5 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Frequency-max: 550kHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 576-3641-5; MIC2169BYMM

Application Information
MOSFET Selection
The MIC2169B controller works from input voltages of
3V to 14.5V and has an internal 5V regulator to provide
power to turn the external N-Channel power MOSFETs
for high- and low-side switches. For applications where
V
mode, and it is necessary that the power MOSFETs
used are sub-logic level and are in full conduction mode
for V
level MOSFETs, whose operation is specified at V
4.5V must be used. For the lower (<5V) applications, the
V
increase the driver voltage to the MOSFET.
It is important to note the on-resistance of a MOSFET
increases with increasing temperature. A 75°C rise in
junction temperature will increase the channel resistance
of the MOSFET by 50% to 75% of the resistance
specified at 25°C. This change in resistance must be
accounted
dissipation and in calculating the value of current-sense
(CS) resistor. Total gate charge is the charge required to
turn the MOSFET on and off under specified operating
conditions (V
the MIC2169B gate-drive circuit. At 500kHz switching
frequency and above, the gate charge can be a
significant source of power dissipation in the MIC2169B.
At low output load, this power dissipation is noticeable
as a reduction in efficiency. The average current
required to drive the high-side MOSFET is:
where:
The low-side MOSFET is turned on and off at V
because the freewheeling diode is conducting during this
time. The switching loss for the low-side MOSFET is
usually negligible. Also, the gate-drive current for the
low-side MOSFET is more accurately calculated using
CISS at V
For the low-side MOSFET:
Since the current from the gate drive comes from the
input voltage, the power dissipated in the MIC2169B due
to gate drive is:
Micrel, Inc.
April 2010
IN
DD
< 5V, the internal V
I
current.
Q
taken from manufacturer’s data sheet for V
I
I
P
G[high-side](avg)
supply can be connected directly to V
[ G
[ G
GS
GATEDRIVE
G
low
high
= total gate charge for the high-side MOSFET
of 2.5V. For applications when V
−
DS
−
side
side
= 0 instead of gate charge.
for
DS
](
](
avg
and V
avg
= average high-side MOSFET gate
=
when
)
)
V
=
=
IN
C
GS
Q
ISS
×
). The gate charge is supplied by
DD
G
(
I
calculating
[ G
×
×
regulator operates in dropout
high
f
V
S
GS
−
side
×
f
S
](
avg
MOSFET
)
+
IN
I
[ G
> 5V; logic-
low
IN
GS
−
to help
= 5V.
side
DS
power
GS
](
= 0
avg
=
)
)
11
A convenient figure of merit for switching MOSFETs is
the on resistance times the total gate charge R
Lower numbers translate into higher efficiency. Low
gate-charge logic-level MOSFETs are a good choice for
use with the MIC2169B.
Parameters that are important to MOSFET switch
selection are:
The voltage ratings for the top and bottom MOSFET are
essentially equal to the input voltage. A safety factor of
20% should be added to the V
to account for voltage spikes due to circuit parasitics.
The power dissipated in the switching transistor is the
sum of the conduction losses during the on-time
(P
the period of time when the MOSFETs turn on and off
(P
where:
Making the assumption the turn-on and turn-off transition
times
approximated by:
where:
The total high-side MOSFET switching loss is:
where:
The low-side MOSFET switching losses are negligible
and can be ignored for these calculations.
CONDUCTION
AC
).
• Voltage rating
• On-resistance
• Total gate charge
R
D = duty cycle =
C
I
P
P
P
t
G
P
t
50ns)
V
f
T
T
S
SW
CONDUCTION
AC
SW
ISS
AC
D
are
= gate-drive current (1.4A for the MIC2169B)
it the switching frequency, nominally 500kHz
= switching transition time (typically 20ns to
=
= freewheeling diode drop, typically 0.5V
= on-resistance of the MOSFET switch
and C
=
C
=
=
) and the switching losses that occur during
P
ISS
P
(
V
AC
equal;
CONDUCTION
IN
×
(
OSS
off
+
V
GS
V
)
are measured at V
=
+
D
I
I
P
)
⎛
⎜ ⎜
⎝
+
G
SW
the
×
AC
V
V
C
I
IN
PK
O
(
OSS
(
rms
on
+
⎞
⎟ ⎟
⎠
×
transition
)
P
)
t
AC
2
×
T
DS
×
V
×
IN
R
(max) of the MOSFETs
f
S
SW
DS
M9999-041210-B
times
= 0
MIC2169B
DS(ON)
can
×Q
be
G
.