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| 型号： | MIC2124YMM |
| 厂商： | Micrel Inc |
| 文件页数： | 12/24页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 10-MSOP |
| 产品培训模块： | MIC2124 Synchronous Current Mode DC-DC Buck Controller |
| 标准包装： | 100 |
| 系列： | Hyper Speed Control™ |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 360kHz |
| 占空比： | 93% |
| 电源电压： | 3 V ~ 5.5 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 10-TFSOP，10-MSOP（0.118"，3.00mm 宽） |
| 包装： | 管件 |
| 其它名称： | 576-3639-5 MIC2124YMM-ND |
P CONDUCTION = I SW(RMS) × R DS(ON)
(8)
Micrel, Inc.
Application Information
MOSFET Selection
The MIC2124 controller works from input voltages of 3V
to 18V and has an external 3V to 5.5V V IN supply to
provide power to turn the external N-Channel power
MOSFETs for the high-side and low-side switches. For
applications where V IN < 5V, it is necessary that the
power MOSFETs used are sub-logic level and are in full
conduction mode for V GS of 2.5V. For applications when
V IN > 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 such as 12V to 1.8V
conversion. 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%,
then the on-resistance 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
(V DS and V GS ). The gate charge is supplied by the
MIC2124 gate-drive circuit. At 300kHz switching
frequency and above, the gate charge can be a
MIC2124
For the low-side MOSFET:
I G[low - side] (avg) = C ISS × V GS × f SW (5)
Since the current from the gate drive comes from the
V IN , the power dissipated in the MIC2124 due to gate
drive is:
P GATEDRIVE = V IN .(I G[high - side] (avg) + I G[low - side] (avg)) (6)
A convenient figure of merit for switching MOSFETs is
the on-resistance times the total gate charge R DS(ON) ×
Q G . Lower numbers translate into higher efficiency. Low
gate-charge logic-level MOSFETs are a good choice for
use with the MIC2124. Also, the R DS(ON) of the low-side
MOSFET will determine the current limit value. Please
refer to “Current Limit” subsection in “Functional
Description” for more details.
Parameters that are important to MOSFET switch
selection are:
? Voltage rating
? On-resistance
? Total gate charge
The voltage ratings for the high-side and low-side
MOSFETs are essentially equal to the power stage input
voltage V HSD . A safety factor of 20% should be added to
the V DS (max) of the MOSFETs to account for voltage
spikes due to circuit parasitic elements.
The power dissipated in the MOSFETs is the sum of the
conduction losses during the on-time (P CONDUCTION ) and
the switching losses during the period of time when the
MOSFETs turn on and off (P AC ).
P SW = P CONDUCTION + P AC (7)
2
significant source of power dissipation in the MIC2124.
At low output load, this power dissipation is noticeable
as a reduction in efficiency. The average current
where:
P AC = P AC(off ) + P AC(on)
(9)
required to drive the high-side MOSFET is:
I G[high - side] (avg) = Q G × f SW
(4)
R DS(ON) = on-resistance of the MOSFET switch
D = Duty Cycle = V OUT / V HSD
t T =
C ISS × V IN + C OSS × V HSD
where:
I G[high-side] (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 IN .
f SW = Switching Frequency (300kHz)
Making the assumption that the turn-on and turn-off
transition times are equal; the transition times can be
approximated by:
(10)
I G
where:
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.
C ISS and C OSS are measured at V DS = 0
I G = gate-drive current
The total high-side MOSFET switching loss is:
P AC = (V HSD + V D ) × I PK × t T × f SW
(11)
June 2010
12
M9999-060810-D
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