MP2905 datasheet(10/16 Pages) MPS

10 / 16 page

MP2905- 3V to 28V INPUT, HYSTERETIC SYNCHRONOUS STEP-DOWN CONTROLLER

MP2905 Rev. 0.91

www.monolithicpower.com

4/18/2011

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider is used to set the

output voltage (see the schematic on front

page).

R1 is for approximately 50µA to 150µA bias

current in the resistor-divider. A wide range of

resistor R1 value is acceptable, choosing a

typical value 6.04k, R3 is determined by:

LOAD

Vout 0.01V (R

0.5 I

)

R3 R1 (

=×

−

where VFB = 0.590V, RDC is the DC resistance

of the output inductor, ILOAD is the full load

current, 0.5*ILOAD is half load condition, it’s for

Load regulation standard. The term 0.01V is to

reflect 1/2 of the feedback threshold hysteresis.

But R3 value also can’t be too large, or circuit

may work abnormally.

Selecting the Inductor

The inductor is required to supply constant

current to the output load while being driven by

the switched input voltage. A larger value

inductor will is favorable to less ripple current

and the lower output ripple voltage. However,

the larger value inductor will have a larger

physical size, higher series DC resistance,

and/or lower saturation current. A good rule for

determining the inductance to use is to allow

the peak-to-peak ripple current in the inductor

to be approximately 30% of the maximum

switch current limit. Also, make sure that the

peak inductor current is below the maximum

switch current limit. The inductance value can

be calculated by:

OUT

)

=× −

×∆

where VIN is the input voltage, VOUT is the output

voltage, fS is the switching frequency, and ∆IL is

the peak-to-peak inductor ripple current.

Choose an inductor that will not saturate under

the maximum inductor peak current. The peak

inductor current can be calculated by:

OUT

LOAD

SIN

)

× −

××

where ILOAD is the full load current.

Setting Current Limit

MP2905 current limit can be set by an external

resistor (R2) which is connected between ILIM

pin and the drain of the high side MOSFET. An

internal 50uA sink current sets a voltage drop

on the resistor. The voltage drop compares to

high-side MOSFET voltage drop (Vds) to set

the peak current limit threshold. Below is the

diagram of current limit function:

ILIM

RILIM

VIN

Rdson_max

High Side

Switch

OC COM

Ids

V+

V-

Figure 2—Current Limit Functional Diagram

The voltage drop on the high side MOSFET is:

DS _ ON

(max) I (max) R

(max)

Where IDS(max) equals the max peak inductor

current ILP (max).

Then, RILIM can be calculated using the

VDS_ON(MAX) with the following formula:

DS _ ON

ILIM

V(max)

R( )

50uA

Ω

And RILIM should be kept in the range of 1kΩ~

8kΩ.

Selecting Power MOSFETs

The MP2905 connects two external, logic-level,

n-channel MOSFETs as the circuit switching

elements. The MOSFETs are the key points for

circuit efficiency. The major parameters we

should concern are:

1) On-resistance, RDS_ON: the lower, the better it

will be.

2) Continuous Drain Current (@10sec), ID: it

should be higher than the peak current @ full

load condition. And pay attention to ID’s

variation with different temperature.

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