MAX8903NETI+T datasheet(16/30 Pages) MAXIM

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MAX8903A–E/G/H/J/N/Y

2A 1-Cell Li+ DC-DC Chargers

for USB and Adapter Power

Maxim Integrated

die temperature. The feedback signal requiring the

smallest current controls the average output current in

the inductor. This scheme minimizes total power dissi-

pation for battery charging and allows the battery to

absorb any load transients with minimum system volt-

age disturbance.

If the battery voltage is below VSYSMIN, the charger does

not directly connect the system voltage to the battery

and the system voltage (VSYS) is slightly above VSYSMIN

as shown in Figure 4. The battery charger independently

controls the battery charging current. VSYSMIN is set to

either 3.0V or 3.4V based on the version of MAX8903_.

See Table 6.

After the battery charges to 50mV above VSYSMIN, the

system voltage is connected to the battery. The battery

fast-charge current then controls the step-down con-

verter to set the average inductor current so that both

the programmed input current limit and fast-charge cur-

rent limit are satisfied.

DC-DC Step-Down Control Scheme

A proprietary hysteretic current PWM control scheme

ensures fast switching and physically tiny external com-

ponents. The feedback control signal that requires the

smallest input current controls the center of the peak

and valley currents in the inductor. The ripple current is

internally set to provide 4MHz operation. When the

input voltage decreases near the output voltage, very

high duty cycle occurs and, due to minimum off-time,

4MHz operation is not achievable. The controller then

provides minimum off-time, peak current regulation.

Similarly, when the input voltage is too high to allow

4MHz operation due to the minimum on-time, the con-

troller becomes a minimum on-time, valley current regu-

lator. In this way, ripple current in the inductor is always

as small as possible to reduce ripple voltage on SYS for

a given capacitance. The ripple current is made to vary

with input voltage and output voltage in a way that

reduces frequency variation. However, the frequency

still varies somewhat with operating conditions. See the

Typical Operating Characteristics.

DC Mode (DCM)

As shown in Table 2, the DC input supports both AC

adapters (up to 2A) and USB (up to 500mA). With the

DCM logic input set high, the DC input is in adapter

mode and the DC input current limit is set by the resis-

tance from IDC to GND (RIDC). Calculate RIDC accord-

ing to the following equation:

RIDC = 6000V/IDC-MAX

With the DCM logic input set low, the DC input current

limit is internally programmed to 500mA or 100mA as

set by the IUSB logic input. With the IUSB logic input

set high, the DC input current limit is 500mA and the

DC input delivers current to SYS through the step-down

regulator. With the IUSB logic input set low, the DC

input current limit is 100mA. In this 100mA mode, the

step-down regulator is turned off and its high-side

switch operates as a linear regulator with a 100mA cur-

rent limit. The linear regulator’s output is connected to

LX and its output current flows through the inductor into

CS and finally to SYS.

The DCM pin has an internal diode to DC as shown in

Figure 1. To prevent current from flowing from DCM

through the internal diode and to the DC input, DCM

cannot be driven to a voltage higher than DC. The

COMPONENT

(FIGURES 2 AND 3)

FUNCTION

PART

CDC, CUSB

Input filter capacitor

4.7µF ceramic capacitor

CVL

VL filter capacitor

1.0µF ceramic capacitor

CSYS

SYS output bypass capacitor

10µF (MAX8903A/MAX8903C/MAX8903D/MAX8903H/MAX8903J) or

22µF (MAX8903B/MAX8903E/MAX8903G/MAX8903Y) ceramic capacitor

CBAT

Battery bypass capacitor

10µF ceramic capacitor

CCT

Charger timing capacitor

0.15µF low TC ceramic capacitor

RPU (X4)

Logic output pullup resistors

100k

Ω

THM

Negative TC thermistor

Philips NTC thermistor, P/N 2322-640-63103, 0k

Ω ±5% at +25°C

THM pullup resistor

10k

Ω

RIDC

DC input current-limit programming resistor 3k

Ω ±1%, for 2A limit

RISET

Fast-charge current programming resistor 1.2k

Ω ±1%, for 1A charging

DC input step-down inductor

1µH inductor with ISAT > 2A

Table 1. External Components List for Figures 2 and 3

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