Rainbow-electronics MAX1870A Manual de usuario descargar pdf (Pagina 26)

MAX1870A

Step-Up/Step-Down

Li+ Battery Charger

26 ______________________________________________________________________________________

LDO provides a 5.4V supply derived from DCIN and

delivers over 10mA. The n-channel MOSFET driver

DBST is powered by DLOV and can source 2.5A and

sink 5A. Since LDO provides power to the internal ana-

log circuitry, use an RC filter from LDO to DLOV as

shown in Figure 1 to minimize noise at LDO. LDO also

supplies the 4.096V reference (REF) and most of the

internal control circuitry. Bypass LDO with a 1µF or

greater capacitor to GND.

Applications Information

Component Selection

Table 4 lists the recommended components and refers

to the circuit of Figure 1. The following sections describe

how to select these components.

MOSFETs

The MAX1870A requires one p-channel MOSFET and

one n-channel MOSFET. Component substitutions are

permissible as long as the on-resistance and gate

charge are equal or lower and the voltage, current, and

power-dissipation ratings are high enough. If using a

lower-power application, scale down the MOSFETs with

lower gate charge and the MOSFET’s on-resistance

can be scaled up. For example, in a system designed

to deliver half as much current, MOSFETs selected with

twice the on-resistance and half as much gate charge

ensure equal or better efficiency, and reduce size and

cost. If resistive losses dominate, it can be possible to

reduce the gate charge at the cost of on-resistance

and still achieve a similar efficiency.

Make sure that the linear regulators can drive the

selected MOSFETs. The average current required to

drive a given MOSFET is:

LDO

= Q

gM2

x f

switch

VHN

= Q

gM1

x f

switch

where f

switch

is 400kHz (typ).

PWM

OGMS

CCS

CLS

CSS

GMS

CSS

CSSP

CSSN/

CSSS

RS1_

Figure 12. CCS Simplified Loop Diagram

CCI LOOP RESPONSE

MAGNITUDE (dB)

-90

-45

-40

-20

100

1k100.1 100k

FREQUENCY (Hz)

CCS LOOP RESPONSE

MAGNITUDE (dB)

-90

-45

-40

-20

100

1k100.1 100k 10M

FREQUENCY (Hz)

PHASE

MAG

Figure 13. CCI and CCS Loop Response

1 2 ... 21 22 23 24 25 26 27 28 29 30 31 32

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Rainbow-electronics MAX1870A Manual de usuario Pagina 26

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