MIC2174 Micrel Semiconductor, MIC2174 Datasheet - Page 12

• Manufacturer Part Number: MIC2174
• Description: Synchronous Buck Controller
• Manufacturer: Micrel Semiconductor

Micrel, Inc.
current and that there is a blanking delay of
approximately 150ns.
where:
V
T
ΔI
D = Duty Cycle
f
The MOSFET R
temperature; therefore, it is recommended to add a 50%
margin to I
limiting due to increased MOSFET junction temperature
rise. It is also recommended to connect LX pin directly to
the drain of the low-side MOSFET to accurately sense
the MOSFETs R
SW
September 2009
OUT
DLY
L(pp)
= Switching frequency
= Current limit blanking time, 150ns typical
= The output voltage
I
CL
= Inductor current ripple peak-to-peak value
=
130mV
CL
R
DS(ON)
in the above equation to avoid false current
ΔI
L(pp)
DS(ON)
+
DS(ON)
=
V
.
OUT
V
OUT
L
×
f
varies 30% to 40% with
SW
T
×
DLY
(1
×
L
−
−
D)
ΔI
L(pp)
2
(3)
(4)
12
MOSFET Gate Drive
The MIC2174 high-side drive circuit is designed to
switch an N-Channel MOSFET. The Block Diagram of
Figure 1 shows a bootstrap circuit, consisting of D1 (a
Schottky diode is recommended) and C
supplies energy to the high-side drive circuit. Capacitor
C
the voltage on the LX pin is approximately 0V. When the
high-side MOSFET driver is turned on, energy from C
is used to turn the MOSFET on. As the high-side
MOSFET turns on, the voltage on the LX pin increases
to approximately V
C
MOSFET on. The bias current of the high-side driver is
less than 10mA so a 0.1μF to 1μF is sufficient to hold
the gate voltage with minimal droop for the power stroke
(high-side switching) cycle, i.e. ΔBST = 10mA x
3.33μs/0.1μF = 333mV. When the low-side MOSFET is
turned back on, C
resistor R
the turn-on time of the high-side N-channel MOSFET.
The drive voltage is derived from the supply voltage VIN.
The nominal low-side gate drive voltage is V
nominal high-side gate drive voltage is approximately V
– V
approximate 30ns delay between the high-side and low-
side driver transitions is used to prevent current from
simultaneously
MOSFETs.
BST
BST
DIODE
is charged, while the low-side MOSFET is on, and
floats high while continuing to keep the high-side
, where V
G
, which is in series with C
DIODE
flowing
BST
HSD
. Diode D1 is reversed biased and
is recharged through D1. A small
is the voltage drop across D1. An
unimpeded
www.DataSheet4U.com
BST
M9999-090409-B
, can slow down
BST
through
. This circuit
MIC2174
IN
and the
both
BST
IN