MAX1908 datasheet(27/30 Pages) MAXIM

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where dV is the maximum voltage sag of 0.5V while

delivering energy to the inductor during the high-side

MOSFET on-time, and dt is the period at highest oper-

ating frequency (400kHz):

Both tantalum and ceramic capacitors are suitable in

most applications. For equivalent size and voltage

rating, tantalum capacitors have higher capacitance,

but also higher ESR than ceramic capacitors. This

makes it more critical to consider ripple current and

power-dissipation ratings when using tantalum capaci-

tors. A single ceramic capacitor often can replace two

tantalum capacitors in parallel.

Output Capacitor

The output capacitor absorbs the inductor ripple cur-

rent. The output capacitor impedance must be signifi-

cantly less than that of the battery to ensure that it

absorbs the ripple current. Both the capacitance and

ESR rating of the capacitor are important for its effec-

tiveness as a filter and to ensure stability of the DC-DC

converter (see the

Compensation section). Either tanta-

lum or ceramic capacitors can be used for the output

filter capacitor.

MOSFETs and Diodes

Schottky diode D1 provides power to the load when the

AC adapter is inserted. This diode must be able to

deliver the maximum current as set by RS1. For

reduced power dissipation and improved dropout per-

formance, replace D1 with a p-channel MOSFET (P1)

as shown in Figure 2. Take caution not to exceed the

maximum VGS of P1. Choose resistors R11 and R12 to

limit the VGS.

The n-channel MOSFETs (N1a, N1b) are the switching

devices for the buck controller. High-side switch N1a

should have a current rating of at least the maximum

charge current plus one-half the ripple current and

have an on-resistance (RDS(ON)) that meets the power

dissipation requirements of the MOSFET. The driver for

N1a is powered by BST. The gate-drive requirement for

N1a should be less than 10mA. Select a MOSFET with a

low total gate charge (QGATE) and determine the

required drive current by IGATE = QGATE

× f (where f is

the DC-DC converter’s maximum switching frequency).

The low-side switch (N1b) has the same current rating

and power dissipation requirements as N1a, and

should have a total gate charge less than 10nC. N2 is

used to provide the starting charge to the BST capacitor

(C15). During the dead time (50ns, typ) between N1a

and N1b, the current is carried by the body diode of

the MOSFET. Choose N1b with either an internal

Schottky diode or body diode capable of carrying the

maximum charging current during the dead time. The

Schottky diode D3 provides the supply current to the

high-side MOSFET driver.

Layout and Bypassing

Bypass DCIN with a 1µF capacitor to power ground

(Figure 1). D2 protects the MAX1908/MAX8724/

MAX8765/MAX8765A when the DC power source input

is reversed. A signal diode for D2 is adequate because

DCIN only powers the internal circuitry. Bypass LDO,

REF, CCV, CCI, CCS, ICHG, and IINP to analog

ground. Bypass DLOV to power ground.

Good PC board layout is required to achieve specified

noise, efficiency, and stable performance. The PC

board layout artist must be given explicit instructions—

preferably, a pencil sketch showing the placement of

the power-switching components and high-current rout-

ing. Refer to the PC board layout in the MAX1908 eval-

uation kit for examples. Separate analog and power

grounds are essential for optimum performance.

Use the following step-by-step guide:

1) Place the high-power connections first, with their

grounds adjacent:

a) Minimize the current-sense resistor trace lengths,

and ensure accurate current sensing with Kelvin

connections.

b) Minimize ground trace lengths in the high-current

paths.

c) Minimize other trace lengths in the high-current

paths.

d) Use > 5mm wide traces.

e) Connect C1 to high-side MOSFET (10mm max

length).

f) LX node (MOSFETs, inductor (15mm max

length)).

Ideally, surface-mount power components are flush

against one another with their ground terminals

almost touching. These high-current grounds are

then connected to each other with a wide, filled zone

of top-layer copper, so they do not go through vias.

The resulting top-layer power ground plane is

connected to the normal ground plane at the

MAX1908/MAX8724/MAX8765/MAX8765As’ back-

side exposed pad. Other high-current paths should

also be minimized, but focusing primarily on short

ground and current-sense connections eliminates

most PC board layout problems.

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