FDMF3170 datasheet(16/21 Pages) ONSEMI

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FDMF3170

Rev. 1.2

Low-Side Driver

The Low-Side Driver (LDRV) is designed to drive the

gate-source of a ground referenced low RDS(ON) N-

channel MOSFET (Q2). The bias for LDRV is internally

connected between PVCC and PGND. When the driver

is enabled, the driver's output is 180 ° out of phase with

the PWM input. When the driver is disabled, LDRV is

held LOW.

Dead-Times

The driver IC design ensures minimum MOSFET dead

times, while eliminating potential shoot-through (cross-

conduction)

currents.

ensure

optimal

module

efficiency, body diode conduction times must be

reduced to the low nano-second range during CCM and

DCM operation. Delay circuitry is added to prevent gate

overlap during both the low-side MOSFET off to high-

side MOSFET on transition and the high-side MOSFET

off to low-side MOSFET on transition.

Exiting 3-State Condition

When exiting a valid 3-state condition, the gate driver of

the FDMF3170 follows the PWM input command. If the

PWM input goes from 3-state to LOW, the low-side

MOSFET is turned on. If the PWM input goes from 3-

state to HIGH, the high-side MOSFET is turned on. This

is illustrated in Figure 26.

Boot Capacitor Refresh

FDMF3170 monitors the low Boot-SW voltage. If EN,

VCC and PVCC are ready, but the voltage across the

boot capacitor voltage is lower than 2.4 V, FDMF3170

ignores the PWM input signal and starts the boot

refresh circuit. The boot refresh circuit turns on the low

side MOSFET with a 100 ns~200 ns narrow pulse in

every 7~14 µs until Boot-SW voltage is above 3.3 V.

Current Monitor (IMON)

The SPS current monitor accurately senses high-side

and low-side MOSFET currents. The currents are

summed together to replicate the output filter inductor

current. The signal is reported from the SPS module in

the form of a 5 µa/A current signal (IIMON-REFIN). The

IMON signal will be referenced to an externally supplied

signal (REFIN) and differentially sensed by an external

analog / digital PWM controller.

The motivation for the IMON feature is to replace the

industry standard output filter DCR sensing, or output

current sense using an external precision resistor. Both

techniques are lossy and lead to reduced system

efficiency. Inductor DCR sensing is also notoriously

inaccurate for low value DCR inductors. Figure 27

shows a comparison between conventional inductor

DCR sensing and the unique IMON feature.

The accuracy on IMON signal is ±5 % from 10 A to 40 A

output current. For the SPS module, parameters that

can affect IMON accuracy are tightly controlled and

trimmed at the MOSFET / IC production stage. The

user can easily incorporate the IMON feature and

accuracy replacing the traditional current sensing

methods in multi-phase VR applications.

The REFIN voltage is an externally supplied DC

voltage. The DC voltage can be supplied from any DC

rail capable of supplying 100 µA, such as a PWM

controller or other power rail in system.

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