MP2632B datasheet(39/43 Pages) MPS

39 / 43 page

MP2632B

– ALL-IN-ONE, 3A SW CHARGER, 3A BOOST

MP2632B Rev. 1.0

www.MonolithicPower.com

3/9/2018

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

For convenience, an NTC thermistor design

spreadsheet is provided. Please contact MPS

for this spreadsheet

Setting the Output Current Limit in Boost

Mode

In boost mode, connect a resistor from OLIM to

AGND to program the output current limit. The

relationship between the output current limit

and the setting resistor is shown in Equation

(12):

OLIM

1500

(A)

(k ) RS1(m )



 



(12)

The output current limit of the boost can be

programmed up to 2.8A (min). Considering a

7% output current limit accuracy, a 3.0A output

current limit is required, typically. According to

Equation (12), given a 10m

Ω sense resistor, a

49.9k

Ω ROLIM can achieve a 3.0A output current

limit.

For safe operation, ROLIM cannot be lower than

49.9

kΩ.

Given a 10mΩ RS1, Table 7 lists the expected

ROLIM values for the typical output current limit.

Table 7: Output Current vs. ROLIM

ROLIM (kΩ)

Output Current (A)

150

1.0

100

1.5

2.0

2.5

49.9

3.0

Selecting the Inductor

Inductor selection is a trade-off between cost,

size, and efficiency. A lower inductance value

results in a smaller size but also has higher

current

ripples,

higher

magnetic

hysteretic

losses,

and

higher

output

capacitances.

However, a higher inductance value results in

lower ripple current and smaller output filter

capacitors but also has higher inductor DC

resistance (DCR) loss. Choose an inductor that

will not saturate under the worst-case load

condition.

Selecting an Inductor in Charge Mode

When the MP2632B works in charge mode (as

buck

converter),

estimate

the

required

inductance with Equation (13):

BATT

L _ MAX







(13)

Where VIN is the typical input voltage, VBATT is

the typical CC charge threshold, fS is the typical

switching

frequency,

and

∆I

L_MAX

the

maximum

peak-to-peak

inductor

current

(usually designed at 30 - 40% of the CC charge

current).

With a typical 5V input voltage, there is a 35%

inductor current ripple at the corner point

between the trickle charge and the CC charge

(VBATT = 3V, ICC = 2.5A) and an inductance of

2.2μH.

Selecting an Inductor in Boost Mode

When the MP2632B is in boost mode (as a

boost converter), the required inductance value

can be calculated with Equation (14), Equation

(15), and Equation (16):

BATT

SYS

BATT

SYS

L _ MAX

)





  

(14)

L _ MAX

BATT(MAX)

(30% 40%) I









(15)

SYS

SYS(MAX)

BATT(MAX)

BATT







(16)

Where VBATT is the minimum battery voltage, fS

is the switching frequency,

∆IL_MAX is the peak-

to-peak inductor ripple current (approximately

30% of the maximum battery current (IBATT(MAX))),

ISYS(MAX) is the system current, and η is the

efficiency.

The worst-case scenario occurs when the

battery voltage is 3V, the typical system voltage

(VSYS) is 5V, the inductance is 1.5µH, and the

efficiency is 90%. This results in a 30% inductor

current ripple.

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 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

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