MP8756 – 26V, 6A, SYNCHRONOUS, STEP-DOWN CONVERTER

MP8756 Rev. 1.1

www.MonolithicPower.com

14

4/13/2017

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2017 MPS. All Rights Reserved.

Over-Current Protection (OCP)

The MP8756 has a cycle-by-cycle over-current

limiting control. The current-limit circuit employs a

valley current-sensing algorithm. The part uses

element. If the magnitude of the current-sense

signal is above the current-limit threshold, PWM

is not allowed to initiate a new cycle.

The trip level is fixed internally. The inductor

current is monitored by the voltage between GND

and SW. GND is used as the positive current

sensing node so that GND should be connected

to the source terminal of the bottom MOSFET.

Since the comparison is done during the HS-FET

off and LS-FET on states, the OC trip level sets

the valley level of the inductor current. Thus, the

can be calculated with Equation (2):



OC

I

I

I _ limit

2

(2)

In an over-current condition, the current to the

load exceeds the current to the output capacitor,

and the output voltage can fall off. As a result,

the

device

encounters

the

under-voltage

protection threshold and hiccup.

Over-/Under-Voltage Protection (OVP/UVP)

The MP8756 monitors the output voltage to

detect over-voltage and under-voltage. Once the

feedback voltage rises higher than 122% of the

feedback voltage, the OVP comparator output

goes high and the circuit turns off the HS-FET

driver. The LS-FET driver turns on, acting as a

current source. The output is then discharged to

remain within the normal range. The MP8756

exits this regulation period when the feedback

voltage falls below 117% of the reference voltage.

When the feedback voltage falls below 75% of

comparator output goes high, and the part

attempts to restart with hiccup mode periodically

for about 50µs if the feedback voltage remains in

this range.

When the feedback voltage falls below 50% of

and the part enters hiccup mode directly after the

comparator and logic delay.

Under-Voltage Lockout (UVLO) Protection

The MP8756 can start up only when VIN is

higher than the under-voltage lockout (UVLO)

rising threshold voltage. The MP8756 shuts down

when VIN is lower than its falling threshold. The

UVLO protection is non-latch off.

If an application requires a higher UVLO, use EN

to adjust the input voltage UVLO by adding two

external resistors (see Figure 5).

It is recommended to use the resistor divider to

set the EN voltage above the EN rising threshold

and below the 4.5V absolute maximum rating.

The rising threshold should be set to provide

enough hysteresis to allow for any input supply

variations.

To avoid an excessive sink current on EN, keep

600kΩ internal pull-down resistor.

Figure 5: Adjustable UVLO

Connecting EN directly to a voltage source

without a pull-up resistor requires limiting the

amplitude of the voltage source. The EN voltage

must not exceed the 4.5V absolute maximum

rating to avoid damaging the IC.

Thermal Shutdown

The MP8756 employs thermal shutdown. The

junction temperature of the IC is monitored

internally. If the junction temperature exceeds the

threshold value (typically 140°C), the converter

shuts off. This is a non-latch protection. There is

a hysteresis of about 25°C. Once the junction

temperature drops to about 125°C, a soft start is

initiated.

Output Discharge

When EN is low, the MP8756 discharges the

output using an internal 6Ω MOSFET.