MP8861 – 2.85V - 18V, 6A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8861 Rev. 1.1

www.MonolithicPower.com

18

5/28/2020

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Input Over-Voltage Protection (VIN OVP)

The MP8861 monitors VIN to detect an input

over-voltage event. This function is active only

when the output is in an over-voltage (OV) or

soft-stop condition. When the output is in over-

voltage protection (OVP), or soft stop is

enabled, output discharge is enabled to charge

the input voltage high. When the input voltage

exceeds the input OVP threshold, both the HS-

FET and LS-FET stop switching.

Output Over-Voltage Protection (OVP)

The MP8861 monitors both FB and VOUT to

detect an over-voltage event. An internal

comparator monitors FB. When the FB voltage

rises higher than 125% of the internal reference

voltage,

the

controller

enters

dynamic

regulation mode, and the input voltage may be

charged up during this time. When input OVP is

triggered, the IC stops switching. If OVP mode

is set to auto-retry in the I2C, the IC begins

switching once the input voltage drops below

the VIN OVP recover threshold. Otherwise, the

MP8861 latches off. OVP auto-retry mode or

latch-off mode occurs only if the soft start has

finished.

Dynamic regulation mode can be operated by

turning on the low side until the low-side

negative current limit is triggered. Then the

body diode of the HS-FET free-wheels the

current.

The output power charges the input, which may

trigger the VIN OVP function. In VIN OVP,

neither the HS-FET or LS-FET turn on and stop

charging VIN. If the output is still over-voltage

and the input voltage drops below the VIN OVP

threshold, repeat the operation. If the output

voltage is below 110% of the internal reference

voltage, then the MP8861 exits output OVP.

Output Absolute Over-Voltage Protection

(OVP_ABS)

The MP8861’s VOUT can be adjusted by the

output reference voltage and the external

resistor dividers. The MP8861’s output voltage

must be lower than the absolute OVP threshold

(typically 6.5V). The MP8861 monitors VOUT to

detect absolute OVP. When VOUT is larger

than 6.5V, the controller enters dynamic

regulation mode if the OVP Retry bit is set to 1

in the I2C register 01. Otherwise, the MP8861

latches off when output OVP and input OVP are

both triggered. Absolute OVP works once both

the input voltage and EN are higher than their

rising thresholds. This means that this function

can work even during a soft start.

Thermal Shutdown

Thermal shutdown prevents the chip from

operating at exceedingly high temperatures.

When the silicon die reaches temperatures that

exceed 160°C, the entire chip shuts down.

When the temperature is less than its lower

threshold (typically 140°C), the chip is enabled

again.

The D[1] and D[2] bits can be monitored in

register 06 for more information about IC silicon

temperature.

Floating Driver and Bootstrap Charging

An external bootstrap capacitor powers the

floating power MOSFET driver. This floating

driver has its own UVLO protection. This

UVLO’s rising threshold is 2.4V with a 150mV

hysteresis. The bootstrap capacitor voltage is

regulated by VIN internally through D1, M1, C4,

L1, and C2 (see Figure 6). If VBST - VSW exceeds

3.3V, U1 regulates M1 to maintain a 3.3V BST

voltage across C4.

Figure 6: Internal Bootstrap Charging Circuit

Start-Up and Shutdown

If VIN, VCC, and EN exceed their respective

thresholds, the chip starts up. The reference

block starts first, generating stable reference

voltages and currents, and then the internal

regulator is enabled. The regulator provides a

stable supply for the remaining circuitries.

Several events can shut down the chip: EN low,

VIN low, VCC low, thermal shutdown, OVP

latch, and OCP latch. In the shutdown

procedure, the signaling path is blocked first to

avoid any fault triggering. The EAO voltage and

the internal supply rail are then pulled down.