MIC24055 Datasheet, PDF (16/29 Pages) Micrel Semiconductor – 12V, 12A High-Efficiency Buck Regulator

Micrel, Inc.
In each switching cycle of the MIC24055 converter, the
inductor current is sensed by monitoring the low-side
MOSFET in the OFF period. If the peak inductor current
is greater than 21A, then the MIC24055 turns off the
high-side MOSFET and a soft-start sequence is
triggered. This mode of operation is called “hiccup mode”
and its purpose is to protect the downstream load in
case of a hard short. The load current-limit threshold has
a fold back characteristic related to the feedback voltage
as shown in Figure 4.
30
25
20
15
10
5
0
0.0
Current Limit Threshold
vs. Feedback Voltage
0.2
0.4
0.6
0.8
1.0
FEEDBACK VOLTAGE (V)
MIC24055
When the low-side MOSFET is turned back on, C_BST is
recharged through D1.
A small resistor R_G, which is in series with C_BST, can be
used to slow down the turn-on time of the high-side N-
channel MOSFET.
The drive voltage is derived from the V_DD supply voltage.
The nominal low-side gate drive voltage is V_DD and the
nominal high-side gate drive voltage is approximately
V_DD – V_DIODE, where V_DIODE 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.
Figure 4. MIC24055 Current-Limit Foldback Characteristic
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_BST. This circuit supplies energy to the high-side drive
circuit. Capacitor C_BST is charged, 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_BST is used to turn the MOSFET
on. As the high-side MOSFET turns on, the voltage on
the SW pin increases to approximately V_IN. Diode D1 is
reverse biased and C_BST 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 1.67μs/0.1μF = 167mV.
December 2012
16
M9999-120712-A