MP2637 datasheet(33/36 Pages) MPS

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MP2637 –2.5A SINGLE CELL SW MODE BATTERY CHARGER WITH PPM AND 2.4A BOOST

MP2637 Rev. 1.04

www.MonolithicPower.com

8/3/2017

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

When the MP2637 is in Boost mode (as a Boost

converter), the required inductance value is

calculated as:

BATT

SYS

BATT

SYS

L _ MAX

)







 

(23)

L _ MAX

BATT(MAX)

(30% 40%) I









(24)

SYS

BATT(MAX)

BATT







(25)

Where VBATT is the minimum battery voltage, fSW

is the switching frequency, and ∆I

L_MAX

is the

peak-to-peak inductor ripple current, which is

approximately 30% of the maximum battery

current IBATT(MAX), ISYS(MAX) is the system current

and η is the efficiency.

In the worst case where the battery voltage is 3V,

a 30% inductor current ripple, and a typical

system voltage (VSYS=5V), the inductance is

1.5µH when the efficiency is 90%.

For best results, use an inductor with an

inductance of 2.2uH with a DC current rating that

is not lower than the peak current of MOSFET.

For higher efficiency, minimize the inductor’s DC

resistance.

Selecting the Input Capacitor CIN

The input capacitor CIN reduces both the surge

current drawn from the input and the switching

noise from the device. The input capacitor

impedance at the switching frequency should be

less than the input source impedance to prevent

high-frequency-switching current from passing to

the

input.

For

best

results,

use

ceramic

capacitors with X7R dielectrics because of their

low ESR and small temperature coefficients. For

most applications, a 22µF capacitor will suffice.

Selecting the System Capacitor CSYS

Select CSYS based on the demand of the system

current ripple.

1. Charge Mode

The capacitor CSYS acts as the input capacitor of

the buck converter in charge mode. The input

current ripple is:

IN _ MAX

RMS _ MAX

SYS _ MAX

IN _ MAX

V )





(26)

2. Boost Mode

The capacitor, CSYS, is the output capacitor of

boost converter. CSYS keeps the system voltage

ripple small and ensures feedback loop stability.

The system current ripple is given by:

SYS _ MAX

RMS _ MAX

SYS _ MAX

V )





(27)

Since the input voltage is passes to the system

directly, VIN_MAX=VSYS_MAX, both charge mode and

boost mode have the same system current ripple.

For

ICC_MAX=2A,

VTC=3V,

VIN_MAX=6V,

the

maximum ripple current is 1.25A. Select the

system capacitors base on the ripple-current

temperature rise not exceeding 10°C. For best

results,

use

ceramic

capacitors

with

X7R

dielectrics with low ESR and small temperature

coefficients. For most applications, use three

22µF capacitors.

Selecting the Battery Capacitor CBATT

CBATT is in parallel with the battery to absorb the

high-frequency switching ripple current.

1. Charge Mode

The capacitor CBATT is the output capacitor of the

buck converter. The output voltage ripple is then:

BATT

SYS

BATT

1 V

/ V

8 C









(28)

2. Boost Mode

The capacitor CBATT is the input capacitor of the

boost converter. The input voltage ripple is the

same as the output voltage ripple from equation

(28)

Both charge mode and boost mode have the

same battery voltage ripple. The capacitor CBATT

can be calculated as:

SYS _ MAX

BATT

BATT _ MAX

1 V

/ V





 



(29)

To guarantee the ±0.5% BATT voltage accuracy,

the maximum BATT voltage ripple must not

exceed 0.5% (e.g. 0.2%). The worst case occurs

at the minimum battery voltage of the CC charge

with the maximum input voltage.

MP2637 Datasheet(PDF) 33 Page - Monolithic Power Systems

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