MAX8815AETB+T Datasheet MAX8815AETB+T | P10-P12

MAX8815A

DC-DC Converter

The MAX8815A uses a current-mode PWM control

scheme. The voltage difference between FB and an

internal 1.265V (typ) reference generates an error sig-

nal that programs the peak inductor current to regulate

the output voltage. The default peak-inductor current

limit is 2.5A (typ). Inductor current is sensed across the

internal switch and summed with a slope-compensation

signal. The PWM comparator compares this summed

signal to the error-amplifier output. At the beginning of

each clock cycle, the n-channel switch turns on until

the PWM comparator trips. During this time, inductor

current ramps up, storing energy in its magnetic field.

When the n-channel switch turns off, the internal syn-

chronous p-channel rectifier turns on. The inductor

releases the stored energy as the current ramps down

and provides energy to the output. The MAX8815A

operates in two modes, normal mode and FPWM mode,

depending on the voltage at SKIPB.

Normal Mode

Drive SKIPB low to select the normal mode of opera-

tion. In this mode, the device operates in PWM only

when driving medium to heavy loads. As the load cur-

rent decreases and crosses the low-power idle-mode

threshold, the PWM comparator and oscillator are dis-

abled. In this low-power mode, switching occurs only

as needed to service the output. This improves the effi-

ciency for light loads, and the device consumes only

30µA under no-load conditions. The threshold for enter-

ing skip mode is approximately 90mA load with a 3.6V

input and 5V output. When switching in normal mode,

the inductor current terminates at zero for each switch-

ing cycle.

FPWM Mode

Drive SKIPB high to select the MAX8815A’s FPWM

mode of operation. The IC switches at a constant fre-

quency (2MHz) and modulates the MOSFET switch

pulse width to control the power transferred per cycle

to regulate the output voltage. Switching harmonics

generated by fixed-frequency operation are consistent

and easily filtered. This is important in noise-sensitive

applications.

The MAX8815A does not allow for dynamic switching

between normal and FPWM modes.

Load-Transient Response/

Voltage Positioning

The MAX8815A matches the load regulation to the volt-

age droop seen during load transients. This is some-

times called voltage positioning. Benefits include lower

peak-to-peak output-voltage deviation for a given load

step without requiring an increase in filter load capaci-

tance. There is minimal voltage droop when transition-

ing from light load to full load and minimum overshoot

when going from full load to light load.

The term “positioning” refers to setting the output volt-

age to a level that is dependent on load current (Figure

2). At minimum load, the output voltage is set to a

slightly higher than nominal level. At full load, the output

voltage is slightly lower than the nominal level. With

voltage positioning, the total voltage deviation during a

transient is significantly improved over traditional high-

gain control loops. Traditional high-gain loops use inte-

grators that maximize gain at low frequencies to

provide tight DC-load regulation; however, due to the

capacitive element in the feedback loop, these high-

gain amplifiers typically take hundreds of microsec-

onds to respond to a load step and return to steady

state. As a result, the voltage can droop by as much as

6% or more during the recovery time. In portable equip-

ment where the output load can change frequently and

the amount of output capacitance is limited, this can

result in a wide short-term output fluctuation (Figure 3).

Voltage positioning on the MAX8815A allows up to 3%

(typ) of load regulation and no further transient droop

(Figures 2 and 3). Thus, during load transients the volt-

age delivered remains within specification more effec-

tively than other regulators that might have tighter DC

accuracy. In systems with high-speed CPUs, thou-

sands of system clock cycles can occur during the time

it takes a traditional high-gain loop to respond to a load

step. Consequently, 3% load regulation with no tran-

sient droop is better suited to such systems than a

power supply that may specify 1% DC-load regulation,

but then exhibits 6% or more of transient droop during

load steps (see the Load Transient graph in the

Typical

Operating Characteristics

section).

1A, 97% Efficiency, 30µA Quiescent Current

Step-Up Converter with True Shutdown

10 ______________________________________________________________________________________

True Shutdown

Drive ON low to place the MAX8815A in shutdown mode

and reduce supply current to 0.1µA (typ).

In shutdown, the control circuitry, internal switching

MOSFET, and synchronous rectifier turn off and LX

becomes high impedance. Drive ON high for normal

operation. The internal synchronous rectifier allows for

conversion efficiencies as high as 97%. In conventional

step-up circuits, the body diode of the synchronous rec-

tifier is forward biased in shutdown and allows current

flow from the battery to the output. If the load cannot be

shut down, an external switch is required to avoid deplet-

ing the battery during shutdown. A proprietary design in

the MAX8815A allows the synchronous rectifier to pro-

vide True Shutdown with no additional components. This

allows the output to fall to GND in shutdown and

removes any connection between the input and output.

Soft-Start

The MAX8815A has internal soft-start circuitry that con-

trols inrush current at startup, reducing transients on

the input source. Soft-start is particularly useful for

higher impedance input sources, such as Li+ and alka-

line cells. The soft-start duration is proportional to the

size of the output capacitor and load resistance. See

the

Typical Operating Characteristics

section for plots

of Soft-Start Time vs. Load Current and Soft-Start Time

vs. Input Voltage.

Fault Protection

The MAX8815A has robust fault and overload protec-

tion. After power-up, the device monitors for an out-of-

regulation state such as an overload or short-circuit

condition. If the converter remains faulted for 16ms, the

output latches off until the part is reinitialized by tog-

gling ON or cycling power to the IC. If the output falls

10% below its regulation voltage or is shorted, the

device enters the fault state immediately.

If the short exists on the output before the IC is pow-

ered up, the converter goes through soft-start once and

then latches off (6ms) because the output never reach-

es regulation. The part draws about 1A of input current

during the startup period. Limiting the time under this

condition prevents thermal runaway.

Applications Information

Setting the Output Voltage

The MAX8815A has a preset output voltage of 5V. To set

other output voltages, use external feedback resistors.

To set the output voltage between 3.3V and 5V, con-

nect FB to the center of an external resistor voltage-

divider between POUT and GND, as shown in Figure

1b. Select the value of R2 no more than 500kΩ, and

then calculate the value for R1 as follows:

R1 = R2 (V

OUT

- 1)

where V

is the FB regulation voltage, 1.265V (typ).

MAX8815A

1A, 97% Efficiency, 30µA Quiescent Current

Step-Up Converter with True Shutdown

______________________________________________________________________________________ 11

(a) HIGH-GAIN DC LOAD REGULATION WITH POOR TRANSIENT RESPONSE

(b) VOLTAGE POSITIONING WITH DC LOAD REGULATION

Figure 3. Transient-Response Comparison

1ms/div

LOAD TRANSIENT

OUT

200mV/div

5V DC OFFSET

500mV/div

MAX8815A toc18

OUT

800mA

10mA

Figure 2. Load Regulation

MAX8815A

1A, 97% Efficiency, 30µA Quiescent Current

Step-Up Converter with True Shutdown

12 ______________________________________________________________________________________

Inductor Selection

In most step-up converter designs, a reasonable induc-

tor value can be derived from the following equation.

This equation sets peak-to-peak inductor current at 1/2

the DC inductor current:

L = (2 x V

BATT

x D x (1 - D))/(I

OUT(MAX)

x f

)

where f

is the switching frequency (2MHz), and D is

the duty factor given by D = 1 - (V

BATT

OUT

). Using L

from the equation above results in a peak-to-peak

inductor current ripple of 0.5 x I

OUT

/(1 - D), and a peak

inductor current of 1.25 x I

OUT

/(1 - D). Ensure that the

peak (saturation) current rating of the inductor meets or

exceeds this requirement. The recommended induc-

tance range for the MAX8815A is 1µH to 2.2µH. See

Table 1 for recommended inductors.

Capacitor Selection

Output Capacitor

Output capacitor C2 in Figures 1a and 1b is required to

keep the output voltage ripple small and to ensure reg-

ulation loop stability. The output capacitors must have

low impedance at the switching frequency. Make sure

the output capacitors maintain their capacitance over

DC bias and the desired operating temperature range.

One 22µF tantalum capacitor is recommended.

Input Capacitor

Input capacitor C1 reduces the current peaks drawn

from the battery or input power source and reduce

switching noise in the IC. The impedance of the input

capacitor at the switching frequency should be kept

very low. A ceramic capacitor is highly recommended

due to their small size and low ESR. Make sure the

input capacitors maintain their capacitance over DC

bias and the desired operating temperature range.

Ceramic capacitors with X5R or X7R dielectric temper-

ature characteristics generally perform well. Two 4.7µF

or one 10µF ceramic capacitors are recommended.

PCB Layout and Routing

Good printed-circuit board (PCB) layout is important to

achieve optimal performance for the MAX8815A. Poor

design can cause excessive conducted and radiated

noise. Conductors carrying discontinuous currents and

any high-current path should be made as short and

wide as possible. Keep the feedback network (R1 and

R2) very close to the IC, preferably within 0.2in of the

FB and GND pins. Nodes with high dV/dt (switching

nodes) should be kept as small as possible and routed

away from FB. Connect the input and output capacitors

as close as possible to the IC. Refer to the MAX8815A

EV kit data sheet for a PCB layout example.

Chip Information

PROCESS: BiCMOS

INDUCTOR

(µH)

DCR

(mΩ)

SAT

(A)

SIZE (mm)

TOKO DE4012CK

A1101AS-1R0M

1 45 3.3 4 x 4 x 1.2

TOKO DE4012CK

A1101AS-2R2M

2.2 60 2.8 4 x 4 x 1.2

TOKO 2818C

1072AS-1R0M

1 40 2.8 2.8 x 2.8 x 1.8

Table 1. Recommended Inductors

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15

MAX8815AETB+T

Mfr. #:

Buy MAX8815AETB+T

Manufacturer:

Maxim Integrated

Description:

Switching Voltage Regulators Step-Up DC/DC Converter

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MAX8815AETB+T