MIC2176-1 MICREL [Micrel Semiconductor], MIC2176-1 Datasheet - Page 16

• Manufacturer Part Number: MIC2176-1
• Description: Wide Input Voltage, Synchronous Buck Controllers Featuring Adaptive On-Time Control
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
Using the typical V
value is roughly estimated as:
For designs where the current ripple is significant
compared to the load current I
operation, calculating the current limit I
into account that one is sensing the peak inductor
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 SW pin directly
to the drain of the low-side MOSFET to accurately sense
the MOSFETs R
SW
November 2010
DLY
OUT
L(pp)
= Switching frequency
= Current-limit blanking time, 150ns typical
= The output voltage
= Inductor current ripple peak-to-peak value
I
I
ΔI
CL
CL
L(pp)
CL
=
≈
in the above equation to avoid false current
R
130mV
R
130mV
=
DS(ON)
DS(ON)
DS(ON)
V
OUT
DS(ON)
CL
f
SW
.
value of 130mV, the current-limit
+
×
V
(1
×
OUT
L
−
varies 30% to 40% with
D)
L
×
OUT
T
DLY
, or for low duty cycle
−
ΔI
CL
L(pp)
2
should take
(3)
(4)
(5)
16
MOSFET Gate Drive
The MIC2176 high-side drive circuit is designed to
switch an N-Channel MOSFET. Figure 1 shows a
bootstrap circuit, consisting of D1 (a Schottky diode is
recommended) and C
the high-side drive circuit. Capacitor C
while the low-side MOSFET is on and the voltage on the
SW pin is approximately 0V. When the high-side
MOSFET driver is turned on, energy from C
turn the MOSFET on. As the high-side MOSFET turns
on,
approximately V
floats high while continuing to keep the high-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. When the low-side MOSFET is
turned back on, C
resistor R
slow down the turn-on time of the high-side N-channel
MOSFET.
The drive voltage is derived from the V
The nominal low-side gate drive voltage is V
nominal high-side gate drive voltage is approximately
V
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.
DD
– V
the
DIODE
G
, which is in series with C
voltage
, where V
IN
. Diode D1 is reverse biased and C
BST
on
BST
is recharged through D1. A small
DIODE
. This circuit supplies energy to
the
is the voltage drop across
SW
BST
pin
DD
M9999-111710-A
, can be used to
BST
supply voltage.
increases
BST
is charged
DD
MIC2176
is used to
and the
BST
to