Rainbow Electronics MAX15026 - Manual (Page 10)

MAX15026
The adaptive driver dead-time allows operation without
shoot-through with a wide range of MOSFETs, minimiz-
ing delays and maintaining efficiency. There must be a
low-resistance, low-inductance path from DL and DH to
the MOSFET gates for the adaptive dead-time circuits
to function properly. The stray impedance in the gate
discharge path can cause the sense circuitry to inter-
pret the MOSFET gate as off while the V
GS of the
MOSFET is still high. To minimize stray impedance, use
very short, wide traces.
Synchronous rectification reduces conduction losses in
the rectifier by replacing the normal low-side Schottky
catch diode with a low-resistance MOSFET switch. The
MAX15026 features a robust internal pulldown transis-
tor with a typical 1
Ω R
DS(ON) to drive DL low. This low
on-resistance prevents DL from being pulled up during
the fast rise time of the LX node, due to capacitive cou-
pling from the drain to the gate of the low-side synchro-
nous rectifier MOSFET.
High-Side Gate-Drive Supply (BST)
and Internal Boost Switch
An internal switch between BST and DH turns on to
boost the gate voltage above V
IN providing the neces-
sary gate-to-source voltage to turn on the high-side
MOSFET. The boost capacitor connected between BST
and LX holds up the voltage across the floating gate
driver during the high-side MOSFET on-time.
The charge lost by the boost capacitor for delivering the
gate charge is replenished when the high-side MOSFET
turns off and LX node goes to ground. When LX is low,
an internal high-voltage switch connected between
V
DRV and BST recharges the boost capacitor. See the
Boost Capacitor section in the Applications Information
to choose the right size of the boost capacitor.
Enable Input (EN), Soft-Start,
and Soft-Stop
Drive EN high to turn on the MAX15026. A soft-start
sequence starts to increase step-wise the reference
voltage of the error amplifier. The duration of the soft-
start ramp is 2048 switching cycles and the resolution
is 1/64th of the steady-state regulation voltage allowing
a smooth increase of the output voltage. A logic-low on
EN initiates a soft-stop sequence by stepping down the
reference voltage of the error amplifier. After the soft-
stop sequence is completed, the MOSFET drivers are
both turned off. See Figure 1.
Connect EN to V CC for always-on operation. Owing to
the accurate turn-on/-off thresholds, EN can be used as
UVLO adjustment input, and for power sequencing
together with the PGOOD output.
When the valley current limit is reached during soft-start
the MAX15026 regulates to the output impedance times
the limited inductor current and turns off after 4096
clock cycles. When starting up into a large capacitive
load (for example) the inrush current will not exceed the
current-limit value. If the soft-start is not completed
before 4096 clock cycles, the device will turn off. The
device remains off for 8192 clock cycles before trying
to soft-start again. This implementation allows the soft-
start time to be automatically adapted to the time nec-
essary to keep the inductor current below the limit while
charging the output capacitor.
Power-Good Output (PGOOD)
The MAX15026 includes a power-good comparator to
monitor the output voltage and detect the power-good
threshold, fixed at 93% of the nominal FB voltage. The
open-drain PGOOD output requires an external pullup
resistor. PGOOD sinks up to 2mA of current while low.
PGOOD goes high (high-impedance) when the regula-
tor output increases above 93% of the designed nomi-
nal regulated voltage. PGOOD goes low when the
regulator output voltage drops to below 90% of the
nominal regulated voltage. PGOOD asserts low during
hiccup timeout period.
Startup into a Prebiased Output
When the MAX15026 starts into a prebiased output, DH
and DL are off so that the converter does not sink cur-
rent from the output. DH and DL do not start switching
until the PWM comparator commands the first PWM
pulse. The first PWM pulse occurs when the ramping
reference voltage increases above the FB voltage.
When the output voltage is biased above the output
set-point, the controller tries to pull the output down to
the set-point once the internal soft-start is complete.
This pulldown is limited by the sink current limit, which
is slowly increased to its normal value to minimize out-
put undershoot.
Low-Cost, Small, 4.5V to 28V Wide Operating
Range, DC-DC Synchronous Buck Controller
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