MIC24052 Datasheet, PDF (19/34 Pages) Micrel Semiconductor – 12V, 6A High-Efficiency Buck Regulator

I_L
0
V_FB
V_REF
ZC
I_L CROSSES 0 AND V_FB > 0.8.
DISCONTINUOUS MODE STARTS.
V_FB < 0.8
WAKE UP FROM DISCONTINUOUS MODE
V
HSD
V_LSD
ESTIMATED ON-TIME
FIGURE 4-3:
MIC24052 Control Loop
Timing (Discontinuous Mode).
During discontinuous mode, the zero crossing
comparator and the current-limit comparator are turned
off. The bias current of most circuits are reduced. As a
result, the total power supply current during
discontinuous mode is only about 450 µA, allowing the
MIC24052 to achieve high efficiency in light load
applications.
4.4 V_DD Regulator
The MIC24052 provides a 5V regulated output for input
voltage V_IN ranging from 5.5V to 19V. When V_IN < 5.5V,
V_DD should be tied to PV_IN pins to bypass the internal
linear regulator.
4.5 Soft-Start
Soft-start reduces the power supply input surge current
at startup by controlling the output voltage rise time.
The input surge appears while the output capacitor is
charged up. A slower output rise time will draw a lower
input surge current.
The MIC24052 implements an internal digital soft-start
by making the 0.8V reference voltage V_REF ramp from
0 to 100% in about 3 ms with 9.7 mV steps. Therefore,
the output voltage is controlled to increase slowly by a
stair-case V_FB ramp. Once the soft-start cycle ends, the
related circuitry is disabled to reduce current
consumption. V_DD must be powered up at the same
time or after V_IN to make the soft-start function
correctly.
4.6 Current Limit
The MIC24052 uses the R_DS(ON) of the internal
low-side power MOSFET to sense overcurrent
conditions. This method will avoid adding cost, board
 2016 Microchip Technology Inc.
MIC24052
space and power losses taken by a discrete current
sense resistor. The low-side MOSFET is used because
it displays much lower parasitic oscillations during
switching than the high-side MOSFET.
In each switching cycle of the MIC24052 converter, the
inductor current is sensed by monitoring the low-side
MOSFET in the OFF period. If the peak inductor
current is greater than 11A, then the MIC24052 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 4-4.
FIGURE 4-4:
MIC24052 Current-Limit
Foldback Characteristic.
4.7 Power Good (PG)
The power good (PG) pin is an open drain output that
indicates logic high when the output is nominally 92%
of its steady state voltage. A pull-up resistor of more
than 10 kΩ should be connected from PG to V_DD.
4.8 MOSFET Gate Drive
The Block Diagram 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 CBST 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
10 mA so a 0.1 μF to 1 μF is sufficient to hold the gate
DS20005659A-page 19