MAX1536 Datasheet, PDF (9/19 Pages) Maxim Integrated Products – 3.6A, 1.4MHz, Low-Voltage, Internal-Switch Step-Down Regulator with Dynamic Output Voltage Control

3.6A, 1.4MHz, Low-Voltage, Internal-Switch Step-
Down Regulator with Dynamic Output Voltage Control
Standard Application Circuit
The MAX1536 standard application circuit (Figure 1)
generates a dynamically adjustable output voltage typical
of graphic processor core requirements. See Table 1
for component selections. Table 2 lists the component
manufacturers.
Detailed Description
The MAX1536 synchronous, current-mode, constant-
off-time, PWM DC-to-DC converter steps down an input
voltage (V_IN) from +3.0V to +5.5V to an output voltage
from +0.7V to V_IN. The MAX1536 output delivers up to
3.6A of continuous current. An internal 54mΩ PMOS
power switch and an internal 47mΩ NMOS synchro-
nous rectifier switch improve efficiency, reduce compo-
nent count, and eliminate the need for an external
Schottky diode (Figure 2).
Modes of Operation
The MAX1536 has two modes of operation: constant-
off-time PWM mode, and pulse-skipping Idle Mode.
The logic level on the SKIP input and the current
through the PMOS switch determine the MAX1536
mode of operation.
Forced-PWM mode keeps the switching frequency rel-
atively constant and is desirable in applications that
must always keep the frequency of conducted and
radiated emissions in a narrow band. Visit Maxim’s
website at www.maxim-ic.com for more information on
how to control electromagnetic interference (EMI).
Pulse-skipping Idle Mode has a dynamic switching fre-
quency under light loads and is desirable in applica-
tions that require high efficiency at light loads.
Forced-PWM Mode (SKIP = V_CC)
Connect SKIP to V_CC to force the MAX1536 to operate
in low-noise, constant-off-time PWM mode. Constant-
off-time PWM architecture provides a relatively con-
stant switching frequency (see the Frequency Variation
with Output Current section). A single resistor (R_TOFF)
sets the PMOS power switch off-time that results in a
switching frequency up to 1.4MHz optimizing perfor-
mance trade-offs in efficiency, switching noise, compo-
nent size, and cost.
PWM mode regulates the output voltage by increasing
the PMOS switch on-time to increase the amount of energy
transferred to the load per cycle. At the end of each off-
time, the PMOS switch turns on and remains on until the
output is in regulation or the current through the switch
increases to the 4.8A current limit. When the PMOS
switch turns off, it remains off for the programmed off-
time (t_OFF), and the NMOS synchronous switch turns on.
The NMOS switch remains on until the end of t_OFF. Since
either the NMOS or the PMOS switch is always on in
PWM mode, the inductor current is continuous.
Idle Mode (SKIP = AGND)
Connect SKIP to AGND to allow the MAX1536 to auto-
matically switch between high-efficiency Idle Mode
under light loads and PWM mode under heavy loads.
The transition from PWM mode to Idle Mode occurs
when the load current is half the Idle Mode current
threshold (600mA typ).
In Idle Mode operation, the switching frequency is
reduced to increase efficiency. The inductor current is
discontinuous in this mode and the MAX1536 only initi-
ates an LX switching cycle when V_FB < V_REFIN. When
V_FB falls below V_REFIN, the PMOS switch turns on and
remains on until output is in regulation and the current
through the switch increases to the Idle Mode current
threshold (600mA typ). When the PMOS switch turns
off, the NMOS synchronous switch turns on and remains
on until the current through the switch decreases to the
zero-cross-current threshold of 200mA.
100% Duty-Cycle Operation
When the input voltage drops near the output voltage,
the LX duty cycle increases until the PMOS switch is on
continuously. The dropout voltage in 100% duty cycle
is the output current multiplied by the on-resistance of
the internal PMOS switch and parasitic resistance in
the inductor. The PMOS switch remains on continuously
as long as the current limit is not reached.
Internal Soft-Start Circuit
Soft-start allows a gradual increase of the current-limit
level at startup to reduce input surge currents. When
the MAX1536 is enabled or powered up, its current-
limit threshold is set to 25% of its final value (25% of
4.8A = 1.2A). The current-limit threshold is increased
by 25% every 256 LX cycles. The current-limit thresh-
old reaches its final level of 4.8A after 768 LX cycles or
when the output voltage is in regulation, whichever
occurs first. Additionally, when V_FB < 0.3 × V_REFIN, the
PMOS switch remains off for the extended off-time of 4 ×
t_OFF. As a result of this soft-start feature, the main output
capacitor charges up relatively slowly. The exact time of
the output rise depends on the nominal switching fre-
quency, output capacitance, and the load current. See
the startup waveforms in the Typical Operating
Characteristics.
Short-Circuit/Overload Protection
The MAX1536 can sustain a constant short circuit or
overload. Under a short-circuit or overload condition,
when V_FB < 0.3 × V_REFIN, the MAX1536 uses an
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