MAX8667/MAX8668
1.5MHz Dual Step-Down DC-DC Converters
with Dual LDOs and Individual Enables
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Detailed Description
The MAX8667/MAX8668 dual step-down converters
with dual low-dropout (LDO) linear regulators are
intended to power low-voltage microprocessors or
DSPs in portable devices. They feature high efficiency
with small external component size. The step-down out-
puts are adjustable from 0.6V to 3.3V (MAX8668) or
factory preset (MAX8667) with guaranteed output cur-
rent of 600mA for OUT1 and 1200mA for OUT2. The
1.5MHz hysteretic-PWM control scheme allows for tiny
external components and reduces no-load operating
current to 100µA (typ) with all regulators enabled. Dual,
low-quiescent-current, low-noise LDOs operate down to
1.7V supply voltage. The MAX8667/MAX8668 have
individual enable inputs for each output to facilitate any
supply sequencing.
Step-Down DC-DC Regulators
(OUT1, OUT2)
Step-Down Regulator Architecture
The MAX8667/MAX8668 step-down regulators are opti-
mized for high-efficiency voltage conversion over a
wide load range, while maintaining excellent transient
response, minimizing external component size, and
minimizing output voltage ripple. The DC-DC convert-
ers (OUT1, OUT2) also feature an optimized on-resis-
tance internal MOSFET switch and synchronous
rectifier to maximize efficiency. The MAX8667/
MAX8668 utilize a proprietary hysteretic-PWM control
scheme that switches with nearly fixed frequency at up
to 1.5MHz allowing for ultra-small external components.
The step-down converter output current is guaranteed
up to 600mA for OUT1 and 1200mA for OUT2.
When the step-down converter output voltage falls below
the regulation threshold, the error comparator begins a
switching cycle by turning the high-side p-channel
MOSFET switch on. This switch remains on until the mini-
mum on-time (t
ON
) expires and the output voltage is in
regulation or the current-limit threshold (I
LIMP_
) is
exceeded. Once off, the high-side switch remains off
until the minimum off-time (t
OFF
) expires and the output
voltage again falls below the regulation threshold.
During this off period, the low-side synchronous rectifi-
er turns on and remains on until either the high-side
switch turns on or the inductor current reduces to the
rectifier-off current threshold (I
LXOFF
= 60mA typ). The
internal synchronous rectifier eliminates the need for an
external Schottky diode.
Input Supply and Undervoltage Lockout
The input voltage range of step-down regulators OUT1
and OUT2 is 2.6V to 5.5V. This supply voltage must be
greater than or equal to the LDO supply voltage (V
IN34
).
A UVLO circuit prevents step-down regulators OUT1
and OUT2 from switching when the supply voltage is
too low to guarantee proper operation. When V
IN12
falls
below 2.4V (typ), OUT1 and OUT2 are shut down.
OUT1 and OUT2 turn on and begin soft-start when
V
IN12
rises above 2.5V (typ).
Soft-Start
When initially powered up, or enabled with EN_, the
step-down regulators soft-start by gradually ramping
up the output voltage. This reduces inrush current dur-
ing startup. See the startup waveforms in the
Typical
Operating Characteristics
section.
Current Limit
The MAX8667/MAX8668 limit the peak inductor current
of the p-channel MOSFET (I
LIMP_
). A valley current limit
is used to protect the step-down regulators during
severe overload and output short-circuit conditions.
When the peak current limit is reached, the internal
p-channel MOSFET turns off and remains off until the
output drops below regulation, the inductor current falls
below the valley current-limit threshold, and the mini-
mum off-time has expired.
Voltage Positioning
The OUT1 and OUT2 output voltages and voltage posi-
tioning of the MAX8668 are set by a resistor network
connected to FB_. With this configuration, a portion of
the feedback signal is sensed on the switched side of
the inductor, and the output voltage droops slightly as
the load current is increased due to the DC resistance
of the inductor. This output voltage droop is known as
voltage positioning. Voltage positioning allows the load
regulation to be set to match the voltage droop during
a load transient, reducing the peak-to-peak output volt-
age deviation during a load transient, and reducing the
output capacitance requirements.
Dropout
As the input voltage approaches the output voltage, the
duty cycle of the p-channel MOSFET reaches 100%. In
this state, the p-channel MOSFET is turned on con-
stantly (not switching), and the dropout voltage is the
voltage drop due to the output current across the on-
resistance of the internal p-channel MOSFET (R
PCH
)
and the inductor’s DC resistance (R
L
):
LDO Linear Regulators (OUT3, OUT4)
The MAX8667/MAX8668 contain two low-dropout linear
regulators (LDOs), OUT3 and OUT4. The LDO output
voltages are factory preset, and each LDO supplies
