MAX8614B 查看數據表（PDF） - Maxim Integrated

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MAX8614B

Dual-Output (+ and -) DC-DC Converters for CCD

Maxim Integrated 

MAX8614A/MAX8614B

Dual-Output (+ and -) DC-DC

Converters for CCD

Inverter

The inverter generates output voltages down to -16V

below VCC. An internal power switch and external catch

diode allow conversion efficiencies as high as 85%.

Control Scheme

Both converters use a fixed-frequency, PWM current-

mode control scheme. The heart of the current-mode

PWM controllers is a comparator that compares the

error-amp voltage-feedback signal against the sum

of the amplified current-sense signal and a slope-

compensation ramp. At the beginning of each clock

cycle, the internal power switch turns on until the PWM

comparator trips. During this time the current in the

inductor ramps up, storing energy in the inductor's

magnetic field. When the power switch turns off, the

inductor releases the stored energy while the current

ramps down, providing current to the output.

Fault Protection

The MAX8614A/MAX8614B have robust fault and over-

load protection. After power-up, the device is set to

detect an out-of-regulation state that could be caused by

an overload or short condition at either output. If either

output remains in overload for more than 100ms, both

converters turn off and the FLT flag asserts low. During a

short-circuit condition longer than 100ms on the positive

output, foldback current limit protects the output. During a

short-circuit condition longer than 100ms on the negative

output, both converters turn off and the FLT flag asserts

low. The converters then remain off until the device is

reinitialized by resetting the controller.

The MAX8614A/MAX8614B also have thermal shutdown.

When the device temperature reaches +170°C (typ), the

device shuts down. When it cools down by 20°C (typ), the

converters turn on.

Enable (ONBST/ONINV)

Applying a high logic-level signal to ONBST/ONINV

turns on the converters using the soft-start and pow-

er-on sequencing described below. Pulling ONBST/

ONINV low puts the IC in shutdown mode, turning off the

internal circuitry. When ONBST/ONINV goes high (or if

power is applied with ONBST/ONINV high), the power-on

sequence is set by SEQ. In shutdown, the device con-

sumes only 0.1µA and both output loads are disconnected

from the input supply.

Soft-Start and Inrush Current

The step-up converter and inverter in the MAX8614A/

MAX8614B feature soft-start to limit inrush current and

minimize battery loading at startup. This is accomplished

by ramping the reference voltage at the input of each error

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amplifier. The step-up reference is ramped from 0 to 1V

(where 1V is the desired feedback voltage for the step-up

converter), while the inverter reference is ramped down

from 1.25V to 0 (where 0 is the desired feedback voltage

for the inverter).

During startup, the step-up converter True-Shutdown load

switch turns on before the step-up-converter reference

voltage is ramped up. This effectively limits inrush current

peaks to below 500mA during startup.

Undervoltage Lockout (UVLO)

The MAX8614A/MAX8614B feature undervoltage-

lockout (UVLO) circuitry, which prevents circuit opera-

tion and MOSFET switching when AVCC is less than the

UVLO threshold (2.55V, typ). The UVLO comparator has

25mV of hysteresis to eliminate chatter due to the source

supply output impedance.

Power-On Sequencing (SEQ)

The MAX8614A/MAX8614B have pin-selectable

internally programmed power-on sequencing. This

sequencing covers all typical sequencing options required

by CCD imagers.

When SEQ = 0, power-on sequence can be indepen-

dently controlled by ONINV and ONBST. When SEQ =

0 and ONINV and ONBST are pulled high, both outputs

reach regulation simultaneously. The inverter is held off

while the step-up True-Shutdown switch slowly turns on to

pull PVP to VCC. The positive output rises to a diode drop

below VCC. Once the step-up output reaches this voltage,

the step-up and the inverter then ramp their respective

references over a period of 7ms. This brings the two out-

puts into regulation at approximately the same time.

When SEQ = 1 and ONBST and ONINV are pulled high,

the step-up output powers on first. The inverter is held off

until the step-up completes its entire soft-start cycle and

the positive output is in regulation. Then the inverter starts

its soft-start cycle and achieves regulation in approxi-

mately 7ms.

True Shutdown

The MAX8614A/MAX8614B completely disconnect the

loads from the input when in shutdown mode. In most

step-up converters, the external rectifying diode and

inductor form a DC current path from the battery to the

output. This can drain the battery even in shutdown if a

load is connected at the step-up converter output. The

MAX8614A/MAX8614B have an internal switch between

the input VCC and the inductor node, PVP. When this

switch turns off in shutdown there is no DC path from

the input to the output of the step-up converter. This load

disconnect is referred to as "True Shutdown." At the

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