Datasheet

MAX618
the output capacitor. Output current is limited by the 2A
MOSFET current limit and the MAX618’s package
power-dissipation limit. See the Maximum Output
Current section for details.
In Idle Mode, the MAX618 improves light-load efficien-
cy by reducing inductor current and skipping cycles to
reduce the losses in the internal switch, diode, and
inductor. In this mode, a switching cycle initiates only
when the error comparator senses that the output volt-
age is about to drop out of regulation. When this
occurs, the NMOS switch turns on and remains on until
the inductor current exceeds the nominal 350mA Idle
Mode current limit.
Refer to Table 1 for an estimate of load currents at which
the MAX618 transitions between PWM and Idle Mode.
Compensation Scheme
Although the higher loop gain of voltage-controlled
architectures tends to provide tighter load regulation,
current-controlled architectures are generally easier to
compensate over wide input and output voltage
ranges. The MAX618 uses both control schemes in par-
allel: the dominant, low-frequency components of the
error signal are tightly regulated with a voltage-control
loop, while a current-control loop improves stability at
higher frequencies. Compensation is achieved through
the selection of the output capacitor (C
OUT
), the inte-
grator capacitor (C
COMP
), and the pole capacitor (C
P
)
from FB to GND. C
P
cancels the zero formed by C
OUT
and its ESR. Refer to the Capacitor Selection section for
guidance on selecting these capacitors.
VL Low-Dropout Regulator
The MAX618 contains a 3.1V low-dropout linear regula-
tor to power internal circuitry. The regulator’s input is IN
and its output is VL. The IN to VL dropout voltage is
100mV, so that when IN is less than 3.2V, VL is typically
100mV below IN. The MAX618 still operates when the
LDO is in dropout, as long as VL remains above the
2.7V undervoltage lockout. Bypass VL with a 4.7μF
ceramic capacitor placed as close to the VL and GND
pins as possible.
28V, PWM, Step-Up DC-DC Converter
6 _______________________________________________________________________________________
SHDN
MAX618
IDLE MODE
CURRENT LIMIT
PWM
CURRENT LIMIT
ERROR
COMPARATOR
250kHz
OSCILLATOR
SLOPE
COMPENSATION
LINEAR
REGULATOR
CURRENT-
SENSE
CIRCUIT
PGND
IN
LX OUT
R
14R
VL
FB
COMP
IN
VL
NMOS
REFERENCE
INTEGRATOR
GND
SHUTDOWN
PWM
LOGIC
THERMAL
SHUTDOWN
Figure 2. Functional Diagram