MIC2164 MICREL [Micrel Semiconductor], MIC2164 Datasheet - Page 14

• Manufacturer Part Number: MIC2164
• Description: Constant Frequency, Synchronous Buck Controllers Featuring Adaptive On-Time Control
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
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
DLY
OUT
L(pp)
= Switching frequency
= Current limit blanking time, 150ns typical
= The output voltage
= Inductor current ripple peak-to-peak value
I
CL
CL
=
in the above equation to avoid false current
ΔI
R
130mV
L(pp)
DS(ON)
DS(ON)
DS(ON)
=
.
V
+
OUT
V
f
varies 30% to 40% with
OUT
SW
⋅
(1
⋅
L
L
−
*
D)
T
DLY
−
ΔI
L(pp)
2
(2)
(3)
14
MOSFET Gate Drive
The MIC2164/-2/-3 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 VHSD. Diode D1 is reversed biased
and C
side 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 for MIC2164. When the low-side
MOSFET is turned back on, C
D1. A small resistor R
slow down 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 VIN and the
nominal high-side gate drive voltage is approximately
VIN – VDIODE, where VDIODE is the voltage drop
across D1. An approximate 30ns delay between the
high-side and low-side driver transitions is used to
prevent current from simultaneously flowing unimpeded
through both MOSFETs.
BST
is charged while the low-side MOSFET is on and
BST
floats high while continuing to keep the high-
G
, which is in series with C
BST
is recharged through
M9999-090409-B
MIC2164/-2/-3
BST
. This circuit
BST
, can
BST