Rainbow Electronics MAX8514 - Manual (Page 16)

MAX8513/MAX8514
Wide-Input, High-Frequency, Triple-Output Supplies
with Voltage Monitor and Power-On Reset
16 ______________________________________________________________________________________
Detailed Description
The MAX8513/MAX8514 combine a step-down DC-DC
converter and two LDOs, providing three output volt-
ages for xDSL modem and set-top box applications.
The switching frequency is set with an external resistor
connected from the FREQ pin to GND, and is
adjustable from 300kHz to 1.4MHz. The main step-
down DC-DC controller operates in a voltage-mode,
pulse-width-modulation (PWM) control scheme. The
MAX8513/MAX8514 include two low-cost LDO con-
trollers capable of delivering current from the DC-DC
main output, an extra winding, the input, or from an
alternate supply voltage. The first LDO controller drives
an external NMOS or NPN with a maximum drive of
7.75V. The second LDO controller provides either a
positive 0.8V to 27V output using an external PNP pass
device, or a negative -1V to -18V output with an exter-
nal NPN pass device.
DC-DC Controller
The MAX8513/MAX8514 step-down DC-DC converters
use a PWM voltage-mode control scheme. An internal
high-bandwidth (25MHz) operational amplifier is used
as an error amplifier to regulate the output voltage. The
output voltage is sensed and compared with an internal
1.25V reference to generate an error signal. The error
signal is then compared with a fixed-frequency ramp
by a PWM comparator to give the appropriate duty
cycle to maintain output-voltage regulation. At the ris-
ing edge of the internal clock and when DL (the low-
side MOSFET gate drive) is at 0V, the high-side
MOSFET turns on. When the ramp voltage reaches the
error-amplifier output voltage, the high-side MOSFET
latches off until the next clock pulse. During the high-
side MOSFET on-time, current flows from the input
through the inductor to the output capacitor and load.
At the moment the high-side MOSFET turns off, the
energy stored in the inductor during the on-time is
released to support the load. The inductor current
ramps down through the low-side MOSFET body diode.
After a fixed delay, the low-side MOSFET turns on to
shunt the current from its body diode for a lower voltage
drop to increase the efficiency. The low-side MOSFET
turns off at the rising edge of the next clock pulse, and
when its gate voltage discharges to zero, the high-side
MOSFET turns on after an additional fixed delay and
another cycle starts.
The MAX8513/MAX8514 operate in forced-PWM mode,
so even under light load the controller maintains a con-
stant switching frequency to minimize noise and possi-
ble interference with system circuitry.
Current Limit
The MAX8513/MAX8514s’ switching regulator senses
the inductor current either through the DC resistance of
the inductor itself for lossless sensing, or through a
series resistor for more accurate sensing. When using
the DC resistance of the inductor, an RC filter circuit is
needed (see R19, R20, and C14 of the Typical
Applications Circuits and the Current-Limit Setting sec-
tion). When peak voltage across the sensing circuit
(which occurs at the peak of the inductor current)
exceeds the current-limit threshold set by ILIM, the
controller turns off the high-side MOSFET and turns on
the low-side MOSFET. The inductor current ramps
down and DH turns on again if the inductor current is
below the current-limit threshold at the next clock
pulse. The MAX8513/MAX8514 current-limit threshold
can be set by two external resistors to be proportional
to the output voltage with an adjustable offset level,
providing foldback current-limit and short-circuit pro-
tection. This feature greatly reduces power dissipation
and prevents overheating of external components dur-
ing an indefinite short-circuit at the output. See the
Foldback Current Limit section for how to set ILIM with
external resistors. The current-limit threshold defaults to
170mV when ILIM is connected to VL, and in this case,
the current limit functions as a constant current limit
only. The LDO controllers do not have current limit and
rely on input current limit for protection.
Synchronous-Rectifier Driver (DL)
Synchronous rectification reduces the conduction loss
in the rectifier by replacing the normal Schottky catch
diode with a low-on-resistance MOSFET switch. The
MAX8513/MAX8514 also use the synchronous rectifier
to ensure proper startup of the boost gate-drive circuit.
High-Side Gate-Drive Supply (BST)
A flying-capacitor boost circuit (see D1 and C3 in the
Typical Applications Circuits) generates the gate-drive
voltage for the high-side N-channel MOSFET. On start-
up, the synchronous rectifier (low-side MOSFET, Q2)
forces LX to ground and charges the boost capacitor
(C3) to V
VL - V DIODE . On the second half-cycle, the
controller turns on the high-side MOSFET by closing an
internal switch between BST and DH. This boosts the
voltage at BST to V
VL - V DIODE + V IN , providing the
necessary gate-to-source voltage to turn on the high-
side N-channel MOSFET.