MIC26901 MICREL [Micrel Semiconductor], MIC26901 Datasheet - Page 17

• Manufacturer Part Number: MIC26901
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
Power Good (PG)
The Power Good (PG) pin is an open drain output which
indicates logic high when the output is nominally 92% of
its steady state voltage. A pull-up resistor of more than
10kΩ should be connected from PG to VDD.
MOSFET Gate Drive
The Block Diagram (Figure 1) shows a bootstrap circuit,
consisting of D1 (a Schottky diode is recommended) and
C
circuit. Capacitor C
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
on. As the high-side MOSFET turns on, the voltage on
the SW pin increases to approximately V
reverse biased and C
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 1.67μs/0.1μF = 167mV. When the low-side
MOSFET is turned back on, C
D1. A small resistor R
be used to 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.
July 2011
DD
BST
. This circuit supplies energy to the high-side drive
– V
DIODE
Figure 4. MIC26901 Current-Limit
, where V
20
16
12
8
4
0
0.0
Foldback Characteristic
0.2
Current Limit Threshold
BST
FEEDBACK VOLTAGE (V)
vs. Feedback Voltage
G
BST
, which is in series with C
DIODE
is charged, while the low-side
BST
0.4
floats high while continuing to
is used to turn the MOSFET
is the voltage drop across
0.6
BST
is recharged through
0.8
DD
1.0
IN
supply voltage.
. Diode D1 is
DD
BST
and the
, can
17
M9999-071311-A
MIC26901