MIC2169B Просмотр технического описания (PDF) - Micrel
Номер в каталоге
Компоненты Описание
производитель
MIC2169B Datasheet PDF : 25 Pages
| |||
Micrel, Inc.
VIN
C2
CIN
0.1µF
VSW
RCS
CS
HSD
LSD
Q1
MOSFET N
Q2
MOSFET N
L1 Inductor
1000pF
VOUT
C1
COUT
200 A
Figure 1. The MIC2169B Current Limiting Circuit
The current limiting resistor RCS is calculated by the
following equation:
RCS
=
RDS(ON)Q1 × IL
200µA
where:
IL
= ILOAD
+
Inductor
Ripple Current
2
Inductor Ripple Current =
VOUT
×
(VIN
VIN
−
×
VOUT
FS × L
)
FS = 500kHz
200µA is the internal sink current to program the
MIC2169B current limit.
The MOSFET RDS(ON) varies 30% to 40% with
temperature; therefore, it is recommended to add a 50%
margin to the load current (ILOAD) in the above equation
to avoid false current limiting due to increased MOSFET
junction temperature rise. It is also recommended to
connect RCS resistor directly to the drain of the top
MOSFET Q1, and the RSW resistor to the source of Q1 to
accurately sense the MOSFETs RDS(ON). To make the
MIC2169B insensitive to board layout and noise
generated by the switch node, a 1.4Ω resistor and a
1000pF capacitor is recommended between the switch
node and GND.
Internal VDD Supply
The MIC2169B controller internally generates VDD for
self biasing and to provide power to the gate drives. This
VDD supply is generated through a low-dropout regulator
and generates 5V from VIN supply greater than 5V. For
supply voltage less than 5V, the VDD linear regulator is
approximately 200mV in dropout. Therefore, it is
recommended to short the VDD supply to the input supply
through a 10Ω resistor for input supplies between 3.0V
to 5V.
MIC2169B
MOSFET Gate Drive
The MIC2169B high-side drive circuit is designed to
switch an N-Channel MOSFET. The Functional Block
Diagram shows a bootstrap circuit, consisting of D1 and
CBST, supplies energy to the high-side drive circuit.
Capacitor CBST is charged while the low-side MOSFET is
on and the voltage on the VSW pin is approximately 0V.
When the high-side MOSFET driver is turned on, energy
from CBST is used to turn the MOSFET on. As the
MOSFET turns on, the voltage on the VSW pin
increases to approximately VIN. Diode D1 is reversed
biased and CBST floats high while continuing to keep the
high-side MOSFET on. When the low-side switch is
turned back on, CBST is recharged through D1. The drive
voltage is derived from the internal 5V VDD bias supply.
The nominal low-side gate drive voltage is 5V and the
nominal high-side gate drive voltage is approximately
4.5V due the voltage drop across D1. An approximate
50ns delay between the high-side and low-side driver
transitions is used to prevent current from
simultaneously flowing unimpeded through both
MOSFETs (shoot-through).
Adaptive gate drive is implemented on the high-side (off)
to low-side (on) driver transition to reduce losses in the
flywheel diode and to prevent shoot-through. This is
operated by detecting the VSW pin; once this pin is
detected to reach 1.5V, the high-side MOSFET can be
assumed to be off and the low side driver is enabled.
Total Power Dissipation and Thermal Considerations
Total power dissipation in the MIC2169B equals the
power dissipation caused by driving the external
MOSFETs plus the quiescent supply current:
PdissTOTAL = PdissSUPPLY + PdissDRIVE
where:
PdissSUPPLY = VDD × IDD
IDD is shown in the “PWM Mode Supply Current” graph in
the Typical Characteristics section of the specification.
PdissDRIVE calculations are shown in the Applications
section of the specification.
The die temperature may be calculated once the total
power dissipation is known:
TJ = TA + PdissTOTAL × θJA
where:
TA is the maximum ambient temperature (°C)
TJ is the junction temperature (°C)
PdissTOTAL is the power dissipation of the
MIC2169B (W)
θJC is the thermal resistance from junction-to-
ambient air (°C/W)
April 2010
9
M9999-041210-B
Share Link: 