MP9186 – 20V, 6A SYNCHRONOUS STEP-DOWN CONVERTER

MP9186 Rev. 1.0

www.MonolithicPower.com

10

2/4/2013

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by connecting LPM pin to a voltage divider from

VCC with 2 external resistors. By program LPM

voltage, efficiency and output ripple can be

traded off and optimized. Higher LPM gives

higher peak inductor current thus more efficient

but more output ripple. The curve of inductor

peak current vs. low power mode voltage for

3.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.1 0.2 0.3 0.4

0.5

0.6

0.7 0.8

Figure 3: Inductor Peak Current vs. Low Power

Mode Voltage

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) protects the chip

from operating at insufficient supply voltage.

The MP9186’s UVLO comparator monitors the

output voltage of the internal regulator, VCC.

The UVLO rising threshold is 4V while its falling

threshold is 3.18V.

External Soft-Start

Connect a capacitor from the soft-start pin to

ground to adjust the soft-start time. When soft-

start begins, an internal 8µA current source

charges the external capacitor. The soft-start

capacitor connects to the non-inverting input of

the error amplifier. The soft-start period lasts

until the voltage on the soft-start capacitor

exceeds the 0.798V reference. At this point the

reference voltage takes over at the non-

inverting error-amplifier input. The soft-start

time can be calculated as follows:

SS

SS

0.798V C (nF)

t (ms)

8 A

×

=

µ

If the output of the MP9186 is pre-biased to a

certain voltage during startup, the IC will disable

the switching of both high-side and low-side

switches until the voltage on the internal soft-

start capacitor exceeds the sensed output

voltage at the FB pin.

Over-Current-Protection

The MP9186 has a hiccup mode over-current

limit for when the inductor current peak value

exceeds the set current limit threshold.

When the output voltage drops below 70% of

the reference, and inductor current exceeds the

current limit, the MP9186 will enter hiccup

mode and retry until the fault is removed. This

is especially useful to ensure system safety

under fault conditions.

The hiccup function is disabled during soft-start

duration.

Thermal Shutdown

Thermal shutdown prevents the chip from

operating at exceedingly high temperatures.

When the silicon die temperature exceeds

150°C, it shuts down the whole chip. When the

temperature falls below its lower threshold,

typically 120°C, the chip is enabled again.

Floating Driver and Bootstrap Charging

An external bootstrap capacitor powers the

floating power MOSFET driver. This floating

driver has its own UVLO protection. The

UVLO’s

rising

threshold

is

2.2V

with

a

hysteresis of 150mV. The bootstrap capacitor

voltage is regulated internally by VIN through

a 5V BST voltage across C4. A 10

resistor

placed between SW and BST capacitor is

strongly recommended to reduce SW spike

voltage and noise.

Figure 4: Internal Bootstrap Charging Circuit