Page 4 / 7

18V 3A 500KHz Synchronous Step-Down Regulator

Functional Block Diagram

VCC REGULATOR

OSCILLATOR

REFERENCE

1M

400k

56pF

1pF

COMPARATOR

ON TIME CONTROL

LOGIC CONTROL

BOOST

REGULATOR

HS

DRIVER

LS

DRIVER

VCC

VCC

IN

EN

FB

GND

SW

BS

CURRENT LIMIT

COMPARATOR

+

+

-

CURRENT SENSE

AMPLIFIER

RSEN

ERROR AMPLIFIER

+

-

+

-

+

-

+

-

VCC REGULATOR

OSCILLATOR

REFERENCE

1M

400k

56pF

1pF

COMPARATOR

ON TIME CONTROL

LOGIC CONTROL

BOOST

REGULATOR

HS

DRIVER

LS

DRIVER

VCC

VCC

IN

EN

FB

GND

SW

BS

CURRENT LIMIT

COMPARATOR

+

+

-

CURRENT SENSE

AMPLIFIER

RSEN

ERROR AMPLIFIER

+

-

+

-

Figure 2. HM8113 Block Diagram

Functions Description

Internal Regulator

The HM8113 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 500KHz 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, 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 0.6V. 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 max to 1.2ms.

Over Current Protection & Hiccup

The HM8113 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 HM8113 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

HM8113