MP1601 Rev. 1.0

www.MonolithicPower.com

14

3/24/2016

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OPERATION

The MP1601 uses constant-on-time control with

an input voltage feed-forward to stabilize the

switching frequency over the full input range. It

achieves 1A of continuous output current from a

2.3V-to-5.5V input voltage range with excellent

load and line regulation. The output voltage can

be regulated as low as 0.6V.

Constant-on-Time Control

Compared to fixed-frequency PWM control,

constant-on-time control offers a simpler control

loop and a faster transient response. By using

an input voltage feed-forward, the MP1601

maintains a nearly constant switching frequency

across the input and output voltage ranges. The

switching pulse on time can be estimated with

Equation (1):

OUT

ON

IN

V

T0.454 s

V





(1)

To prevent inductor current runaway during the

load transient, the MP1601 uses a fixed

minimum off time of 60ns.

Sleep Mode Operation

The MP1601 uses sleep mode to achieve high

efficiency at extremely light loads. In sleep

mode, most of the circuit blocks are turned off,

except the error amplifier and the PWM

comparator. Therefore, the operating current is

reduced to a minimal value (see Figure 2).

Figure 2: Operation Blocks in Sleep Mode

When the load becomes lighter, the output

voltage ripple is bigger and drives the error

amplifier output (EAO) lower. When the EAO

hits an internal low threshold, it clamps at that

level, and the MP1601 enters sleep mode.

During sleep mode, the valley of the FB voltage

is regulated to the internal reference voltage,

making the average output voltage slightly

higher than the output voltage at DCM or CCM.

The on-time pulse in sleep mode is around 40%

larger than that in DCM or CCM. Figure 3

shows the average FB voltage relationship with

the internal reference at sleep mode.

Figure 3: FB Average Voltage at Sleep Mode

When the MP1601 is in sleep mode, the

average output voltage is higher than the

internal reference voltage. The EAO is kept low

and clamped in sleep mode. When the loading

increases,

the

PWM

switching

period

decreases to keep the output voltage regulated,

and the output voltage ripple decreases as well.

Once the EAO is higher than the internal low

threshold, the MP1601 exits sleep mode and

enters DCM or CCM, depending on the load. In

DCM or CCM, the EAO regulates the average

output voltage to the internal reference (see

Figure 4).

Figure 4: DCM Control

There is always a loading hysteresis when

entering and exiting sleep mode due to the

error amplifier clamping response time.

AAM Operation at Light-Load Operation

The MP1601 uses an advanced asynchronous

modulation (AAM) power-save mode with a

zero-current cross detection (ZCD) circuit for

light loads.

The MP1601 uses AAM power-save mode in

light loads (see Figure 5). The AAM current

decided by an on-time generator and AAM

comparator. At light-load condition, the SW on

the pulse time is stretched. The AAM

comparator pulse is longer than the on-time

generator. The mode of operation is below in

Figure 6.