Rainbow Electronics MAX15051 User Manual

Page 10

MAX15050/MAX15051

High-Efficiency, 4A, 1MHz, Step-Down Regulators
with Integrated Switches in 2mm x 2mm Package

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turns off when the ramp voltage exceeds the V

COMP

signal or the current-limit threshold is exceeded. The
low-side switch then turns on for the remainder of the
oscillator cycle.

Skip Mode (MAX15050)

The MAX15050 features a skip function. In skip mode,
the MAX15050 switches only as necessary to maintain
the output at light loads (not capable of sinking current
from the output). This maximizes light-load efficiency
and reduces the input quiescent current.

In skip mode, the low-side switch is turned off when the
inductor current decreases to 0.2A (typ) to ensure no
reverse current flowing from the output capacitor.

The high-side switch minimum on-time is controlled to
guarantee that 0.9A current is reached to avoid high
frequency bursts at no-load conditions, which prevents
a rapid increase of the supply current caused by addi-
tional switching losses. Under heavy load, the device
operates as a PWM converter.

Current Limit

The internal, high-side MOSFET has a typical 8A peak
current-limit threshold. When current flowing out of LX
exceeds this limit, the high-side MOSFET turns off and
the low-side MOSFET turns on. The low-side MOSFET
remains on until the inductor current falls below the low-
side current limit. This lowers the duty cycle and caus-
es the output voltage to droop until the current limit is
no longer exceeded. The MAX15050/MAX15051 use a
hiccup mode to prevent overheating during short-cir-
cuit output conditions.

During current limit, if V

drops below 70% of

REFIN/SS

and stays below this level for typically 36µs

or more, the device enters hiccup mode. The high-side
MOSFET and the low-side MOSFET turn off and both
COMP and REFIN/SS are internally pulled low. The
device remains in this state for 896 clock cycles and
then attempts to restart for 112 clock cycles. If the fault-
causing current limit has cleared, the device resumes
normal operation. Otherwise, the device reenters hic-
cup mode.

Soft-Start and Reference Input (REFIN/SS)

The MAX15050/MAX15051 utilize an adjustable soft-
start function to limit inrush current during startup. An
8µA (typ) current source charges an external capacitor
connected to REFIN/SS. The soft-start time is adjusted
by the value of the external capacitor from REFIN/SS to
GND. The required capacitance value is determined as:

where t

is the required soft-start time in seconds.

Connect a minimum 1nF capacitor between REFIN/SS
and GND. REFIN/SS is also an external reference input
(REFIN/SS). The device regulates FB to the voltage
applied to REFIN/SS. The internal soft-start is not avail-
able when using an external reference. Figure 2 shows
a method of soft-start when using an external refer-
ence. If an external reference is not applied, the device
uses the internal 0.6V reference.

Undervoltage Lockout (UVLO)

The UVLO circuitry inhibits switching when V

below 2.55V (typ). Once V

rises above 2.6V (typ),

UVLO clears and the soft-start function activates. A
50mV hysteresis is built-in for glitch immunity.

BST

The gate-drive voltage for the high-side, n-channel
switch is generated by a flying-capacitor boost circuit.
The capacitor between BST and LX is charged from the
V

supply while the low-side MOSFET is on. When the

low-side MOSFET is switched off, the voltage of the
capacitor is stacked above LX to provide the necessary
turn-on voltage for the high-side internal MOSFET.

Power-Good Output (PWRGD)

PWRGD is an open-drain output that goes high
impedance when V

is above 92.5% x V

REFIN/SS

and

REFIN/SS

is above 0.54V. PWRGD pulls low when V

is below 90% of V

REFIN/SS

for at least 48 clock cycles

or V

REFIN/SS

is below 0.54V. PWRGD is low during

shutdown.

0 6

REFIN/SS

MAX15050
MAX15051

Figure 2. Typical Soft-Start Implementation with External
Reference