MP2639A datasheet(32/35 Pages) MPS

32 / 35 page

MP2639A

– SWITCH-MODE BATTERY CHARGER W/ BIDIRECTIONAL OPERATION

MP2639A Rev. 1.0

www.MonolithicPower.com

6/6/2017

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

IVH(MAX) is the battery current (2.5A), and η is the

efficiency.

Under most application conditions, the charge

current is limited at the input current limit, so

IVL(MAX) is 3A, typically.

In the worst-case scenario with a 8.4V battery

voltage, a 30% inductor current ripple, and a

typical input voltage (VVL = 5V), the inductance

is calculated as 1.9µH.

When the MP2639A works in discharge mode

(as a buck converter), estimate the required

inductance with Equation (30):







L _ MAX

(30)

Where VVH is the output voltage, VIN is the input

voltage, fSW is the switching frequency, and

∆I

L_MAX is the maximum peak-to-peak inductor

current (usually 30 - 40% of the discharge

current).

With a typical 8.4V input voltage (2-cell battery),

a 30% inductor current ripple at the max output

current when VVL is set at the typical 5V value

(VVL = 5V, IVL(max) = 5A), and the inductance is

calculated as 1.2µH.

For best results, use an inductor with an

inductance of 2.2µH with a DC current rating no

lower than the peak current of the MOSFET.

For higher efficiency, minimize the inductor’s

DC resistance.

Selecting the VL Capacitor (CVL)

Select the VL capacitor (CVL) based on the

demand of the system current ripple.

CVL is the input capacitor of the boost converter

during charge mode and the output capacitor of

the buck converter during discharge mode.

Calculate its values with Equation (31) and

Equation (32):









1 V

/ V

8 C

(31)





 



VH _ MAX

VL _ MAX

1 V / V

(32)

Suppose that the maximum VL ripple must not

exceed 1% (e.g.: 0.5%). When VVH_MAX is 8.4V,

VVL is 5V, L is 2.2µH, fSW is 1200kHz, and

∆rVL_AX is 0.5%, then CVL is 3.2µF.

One

4.7µF

ceramic

capacitor

with

X7R

dielectrics is sufficient.

Selecting the VH Capacitor (CVH)

The 2-cell battery is connected to the VH port,

which is the output of the boost during charge

mode and the input of the buck converter during

discharge mode.

In discharge mode, the capacitor CVH acts as

the input capacitor of the buck converter. The

input current ripple can be calculated with

Equation (33):





VH _ MAX

RMS _ MAX

VH _ MAX

V )

(33)

In boost mode, the capacitor (CVH) is the output

capacitor of the boost converter. CVH keeps the

VH ripple small (<0.5%) and ensures feedback

loop stability. The VH current ripple is given by

Equation (29).

When IVH_MAX is 2.0A, VVL is 5V, and VVH_MAX is

8.4V, the maximum ripple current is 1A. Select

the system capacitors base on the ripple-

current temperature rise, not to exceed 10°C.

For best results, use X7R dielectric ceramic

capacitors with low ESR and small temperature

coefficients. For most applications, place two

22µF capacitors and one 1µF capacitor as

close to the IC as possible.

MP2639A Datasheet(PDF) 32 Page - Monolithic Power Systems

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