MIC2101_13 Datasheet, PDF (24/36 Pages) Micrel Semiconductor – 38V, Synchronous Buck Controllers Featuring Adaptive On-Time Control

Micrel, Inc.
Application Information
Setting the Switching Frequency
The MIC2101/02 are adjustable-frequency, synchronous
buck controllers featuring a unique adaptive on-time
control architecture. The switching frequency can be
adjusted between 200kHz and 600kHz by changing the
resistor divider network consisting of R19 and R20.
Figure 5. Switching Frequency Adjustment
The following formula gives the estimated switching
frequency:
f_{SW_ADJ}
f_O
u
R20
R19  R20
Eq. 4
Where f_O = Switching Frequency when R19 is 100k and
R20 being open, f_O is typically 600kHz. For more precise
setting, it is recommended to use the following graph:
Switching Frequency
700.00
600.00
500.00
R19 = 100k, IOUT =12A
VIN = 12V
400.00
300.00
VIN =38V
200.00
100.00
0.00
10.00
100.00
1000.00
R20 (k Ohm)
10000.00
Figure 6. Switching Frequency vs. R20
MOSFET Selection
The MIC2101/02 controllers work from input voltages of
MIC2101/02
4.5V to 38V and has internal 5V V_DD LDO. This internal
V_DD LDO provides power to turn the external N-Channel
power MOSFETs for the high-side and low-side
switches. For applications where V_DD < 5V, it is
necessary that the power MOSFETs used are sub-logic
level and are in full conduction mode for VGS of 2.5V. For
applications when V_DD > 5V; logic-level MOSFETs,
whose operation is specified at V_GS = 4.5V must be
used.
There are different criteria for choosing the high-side and
low-side MOSFETs. These differences are more
significant at lower duty cycles. In such an application,
the high-side MOSFET is required to switch as quickly
as possible to minimize transition losses, whereas the
low-side MOSFET can switch slower, but must handle
larger RMS currents. When the duty cycle approaches
50%, the current carrying capability of the high-side
MOSFET starts to become critical.
It is important to note that 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 for when calculating MOSFET power
dissipation and in calculating the value of current limit.
Total gate charge is the charge required to turn the
MOSFET on and off under specified operating conditions
(VDS and VGS). The gate charge is supplied by the
MIC2101/02 gate-drive circuit. At 600kHz switching
frequency, the gate charge can be a significant source of
power dissipation in the MIC2101/02. 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:
I_G[HIGH-SIDE](AVG) Q_G u f_SW
Eq. 5
where:
I_G[HIGHSIDE](avg) = Average high-side MOSFET gate
current
Q_G = Total gate charge for the high-side MOSFET taken
from the manufacturer’s data sheet for V_GS = V_DD.
f_SW = Switching Frequency
The low-side MOSFET is turned on and off at V_DS = 0
because an internal body diode or external 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 C_ISS at V_DS = 0 instead of
gate charge.
November 13, 2013
24
Revision 2.0