18V 2A 500KHz Synchronous Step-Down Regulator

Functions Description

Internal Regulator

The HM1470 is a current mode step down DC/DC converter that provides excellent transient response with no

extra external compensation components. This device contains an internal, low resistance, high voltage power

MOSFET, and operates at a high 500K operating frequency to ensure a compact, high efficiency design with

excellent AC and DC performance.

Error Amplifier

The error amplifier compares the FB pin voltage with the internal FB reference (VFB) and outputs a current

proportional to the difference between the two. This output current is then used to charge or discharge the internal

compensation network to form the COMP voltage, which is used to control the power MOSFET current. The

optimized internal compensation network minimizes the external component counts and simplifies the control loop

design.

Internal Soft-Start

The soft-start is implemented to prevent the converter output voltage from overshooting during startup. When the

chip starts, the internal circuitry generates a soft-start voltage (SS) ramping up from 0V to 0.8V. When it is lower

than the internal reference (REF), SS overrides REF so the error amplifier uses SS as the reference. When SS is

higher than REF, REF regains control. The SS time is internally fixed to 1.2ms.

Over Current Protection & Hiccup

The HM1470 has cycle-by-cycle over current limit when the inductor current peak value exceeds the set current

limit threshold. Meanwhile, output voltage starts to drop until FB is below the Under-Voltage (UV) threshold,

typically 25% below the reference. Once a UV is triggered, the HM1470 enters hiccup mode to periodically restart

the part. This protection mode is especially useful when the output is dead-short to ground. The average short circuit

current is greatly reduced to alleviate the thermal issue and to protect the regulator. The HM1470 exits the hiccup

mode once the over current condition is removed.

Startup and Shutdown

If both VIN and EN are higher than their appropriate thresholds, the chip starts. The reference block starts first,

generating stable reference voltage and currents, and then the internal regulator is enabled. The regulator provides

stable supply for the remaining circuitries. Three events can shut down the chip: EN low, VIN low and thermal

shutdown. In the shutdown procedure, the signaling path is first blocked to avoid any fault triggering. The COMP

voltage and the internal supply rail are then pulled down. The floating driver is not subject to this shutdown

command.

HM1470