MAX15021 Datasheet, PDF (12/24 Pages) Maxim Integrated Products – Dual, 4A/2A, 4MHz, Step-Down DC-DC Regulator with Tracking/Sequencing Capability

MAX15021
Dual, 4A/2A, 4MHz, Step-Down DC-DC
Regulator with Tracking/Sequencing Capability
Error Amplifier
The output of the internal voltage-mode error amplifier
(COMP_) is provided for frequency compensation (see
the Compensation-Design Guidelines section). FB_ is
the inverting input of the error amplifier. The error
amplifier has an 80dB open-loop gain and a 12MHz
gain bandwidth (GBW) product.
Output Short-Circuit
Protection (Hiccup Mode)
The MAX15021 features lossless, high-side peak cur-
rent limit and low-side, valley current limit. At short duty
cycles, both limits are active. At high duty cycles, only
the high-side peak current limit is active. Either limit
causes the hiccup mode count (N_CL) to increment.
For duty cycles less than 50%, the low-side valley cur-
rent limit is active. Once the high-side MOSFET turns off,
the voltage across the low-side MOSFET is monitored. If
this voltage does not exceed the current-limit threshold
at the end of the cycle, the high-side MOSFET turns on
normally at the start of the next cycle. If the voltage
exceeds the current-limit threshold just before the
beginning of a new PWM cycle, the controller skips that
cycle. During severe overload or short-circuit condi-
tions, the switching frequency of the device appears to
decrease because the on-time of the low-side MOSFET
extends beyond a clock cycle.
If the current-limit threshold is exceeded for more than
four cumulative clock cycles (N_CL), the device shuts
down for 8192 clock cycles (hiccup timeout) and then
restarts with a soft-start sequence. If three consecutive
cycles pass without a current-limit event, the count of
N_CL is cleared (see Figure 3). Hiccup mode protects
the device against a continuous output short circuit.
The internal current limit is constant from 5.5V down to
3V and decreases linearly by 50% from 3V to 2V. See
the Electrical Characteristics table.
Thermal-Overload Protection
The MAX15021 features an integrated thermal-overload
protection with temperature hysteresis. Thermal-over-
load protection limits the total power dissipation in the
device and protects it in the event of an extended ther-
mal fault condition. When the die temperature exceeds
+160°C, an internal thermal sensor shuts down the
device, turning off the internal power MOSFETs and
allowing the die to cool. After the die temperature falls
by +15°C, the part restarts with a soft-start sequence.
CURRENT LIMIT
IN _{COUNT OF 4}
N_CL
CLR
INITIATE HICCUP
TIMEOUT
N_HT
IN
COUNT OF 3
N_CLR
CLR
Figure 3. Hiccup-Mode Block Diagram
Startup into a Prebiased Output
(Sequencing Mode)
In sequencing mode, the regulators start into a prebi-
ased output and soft-stop is disabled. During soft-start,
the complementary switching sequence is inhibited until
the PWM comparator commands its first PWM pulse.
Until then, the converters do not sink current from the
outputs. The first PWM pulse occurs when the ramping
reference voltage increases above the FB_ voltage.
PWM Controllers
Design Procedure
Setting the Switching Frequency
Connect a 4.2kΩ to 33kΩ resistor from RT to SGND to
program the switching frequency from 500kHz to
4MHz. Calculate the resistor connected to RT using the
following equation:
R_T[kΩ]
=
f_SW[kHz] × 1.067[V]
32[μA] × 4[MHz]
Higher frequencies allow designs with lower inductor
values and less output capacitance. At higher switch-
ing frequencies core losses, gate-charge currents, and
switching losses increase. When operating from V_AVIN
≤ 3V, the switching frequency (f_SW) should be derated
to 3MHz (maximum).
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Maxim Integrated