MP9840 – 36V, 3.5A, LOW IQ, SYNC STEP-DOWN CONVERTER

OPERATION

The MP9840 is a synchronous, step-down,

switching regulator with integrated, internal,

high-side and low-side power MOSFETs. The

MP9840 provides 3.5A of highly efficient output

current with current mode control.

The MP9840 features a wide input voltage

range, switching frequency programmable from

350kHz to 2.5MHz, external soft start, and

precision current limit. Its very low operational

quiescent current makes it suitable for battery-

powered applications.

Pulse-Width Modulation (PWM) Control

At

moderate-to-high

output

currents,

the

MP9840 operates in a fixed-frequency, peak-

current-control mode to regulate the output

voltage. A pulse-width modulation (PWM) cycle

is initiated by the internal clock. At the rising

edge

of

the

clock,

the

high-side

power

MOSFET (HS-FET) is turned on and remains

on until its current reaches the value set by the

COMP voltage (VCOMP). If the current in the HS-

FET does not reach VCOMP in one PWM period,

the HS-FET remains on, saving a turn-off

operation.

When the high-side power switch is off, the low-

side

MOSFET

(LS-FET)

is

turned

on

immediately and remains on until the next cycle

begins.

For each turn-on and -off in a switching cycle,

the HS-FET turns on and off with a minimum on

and off time limit.

Advanced Asynchronous Mode (AAM)

The MP9840 employs advanced asynchronous

mode (AAM) functionality to optimize efficiency

during light-load or no-load conditions. AAM

can be enabled by connecting SYNC to a low

level (<2V) before start-up; AAM can be

disabled when connecting SYNC to a high level

before

start-up.

SYNC

can

be

used

to

synchronize switching again after start-up.

If

continuous

conduction

mode

(CCM)

is

enabled, the device is forced to work with a

fixed frequency regardless of the output load

current.

The

advantage

of

CCM

is

the

controllable frequency and smaller output ripple,

but it also has low efficiency at light load (see

Figure 2).

If AAM is enabled, the MP9840 first enters non-

synchronous operation for as long as the

inductor current is approaching zero at light

load. If the load is further decreased or is at no

load, VCOMP drops below the AAM voltage

(VAAM), making the MP9840 enter power-save

mode (PSM). This puts the chip into sleep

mode, which consumes very low quiescent

current to further improve light-load efficiency.

In PSM, the internal clock is reset whenever

VCOMP crosses over VAAM, and the crossover

time is taken as the benchmark of the next

clock. When the load increases, and the DC

value of VCOMP is higher than VAAM, the

operation mode is discontinuous conduction

mode (DCM) or CCM, which have a constant

switching frequency.

AAM

(AAM = Low)

Inductor

Current

t

t

t

Load

Decreased

Forced CCM

(AAM = High)

Inductor

Current

t

t

t

Load

Decreased

Figure 2: AAM and Forced CCM

Error Amplifier (EA)

The error amplifier (EA) compares the FB

voltage with the internal reference (0.8V) and

outputs a current proportional to the difference

between the two. This output current is used to

charge or discharge the internal compensation

network to form VCOMP, which is used to control

the power MOSFET current. The optimized

internal compensation network minimizes the

external component count and simplifies the

control loop design.

Bootstrap Charging

The bootstrap capacitor (0.1µF to 1µF) is

charged and regulated to about 5V by the

dedicated internal bootstrap regulator. When

the voltage between the BST and SW nodes is

lower

than

its

regulation,

a

PMOS

pass

transistor connected from VIN to BST is turned

on. The charging current path is from VIN to

BST to SW. An external circuit should provide

enough voltage headroom to facilitate charging.

MP9840 Rev. 1.01

www.MonolithicPower.com

17

1/23/2018

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