MAX6877 データシートの表示（PDF） - Maxim Integrated

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MAX6877

Dual-/Triple-Voltage, Power-Supply Trackers/Sequencers/Supervisors

Maxim Integrated 

Dual-/Triple-Voltage, Power-Supply

Trackers/Sequencers/Supervisors

During fault conditions, an internal pulldown resistor

(100Ω) on OUT_ is enabled to help discharge load

capacitance (100Ω is connected for fast power-down

control).

Outputs

GATE_

The MAX6877/MAX6878/MAX6879 feature up to three

GATE_ outputs to drive up to three external n-channel

FET gates. The following conditions must be met before

GATE_ begins enhancing the external n-channel FET_:

1) All SET_ inputs (SET1–SET3) are above their 0.5V

thresholds.

2) At least one IN_ input or VCC is above the minimum

operating voltage (2.7V).

3) EN/UV > 1.25V.

At power-up mode, GATE_ voltages are enhanced by

control loops so that all OUT_ voltages track together at

a capacitor-adjusted slew rate. Each GATE_ is internal-

ly pulled up to 5V above its relative IN_ voltage to fully

enhance the external n-channel FET when power-up is

complete.

FAULT

The MAX6877/MAX6878/MAX6879 include an open-

drain, active-low tracking fault alarm output (FAULT).

FAULT asserts low when a power-up phase is not com-

pleted within the specified fault period or if OUT_ volt-

ages are more than VTRK_F.

The fault time period (tFAULT) is set through the capaci-

tor at SLEW (CSLEW). Use the following formula to esti-

mate the fault timeout period:

tFAULT = 2.191 x 108 x CSLEW

Power-supply tracking operation should be completed

within the selected fault timeout period (tFAULT). The

total tracking time is extended when the devices must

vary the control slew rate to allow slow supplies to

catch up. If the external FET is too small (RDS is too

high for the selected load current and IN_ source cur-

rent), the OUT_ voltage may never reach the control

ramp voltage. For a slew rate of 935V/s, a fault is sig-

naled if all outputs have not stabilized within 22ms. For

a slew rate of 93.5V/s, a fault is signaled if tracking

takes too long (more than 219ms).

During power-up, only the condition, OUT_ < ramp -

VTRK, is monitored in order to stop the ramp. However,

both conditions OUT < ramp - VTRK_F and OUT_ >

ramp + VTRK_F cause a fault. During power-down, only

the condition OUT > ramp + VTRK is checked in order

to stop the ramp. However, both conditions OUT_ <

ramp - VTRK_F and OUT_ > ramp + VTRK_F cause a

fault (see Figure 10). OUT1, OUT2, and OUT3 are

tracked within VTRK_F (mV) (power-up tracking and

power-down), and if they differ by more than 2 x VTRK_F

a fault condition is asserted.

Retry time period (tRETRY) is defined as 16 x tFAULT. To

calculate the retry time period use the following formula:

tRETRY = 3.506 x 109 x CSLEW

where tRETRY is in µs and CSLEW is in farads.

Autoretry and Latch-Off Functions (LTCH/RTR)

The MAX6877/MAX6878/MAX6879 feature latch-off or

autoretry modes to power-on again after a fault condi-

tion has been detected. Connect LTCH/RTR to ground

to set the latch-off mode. To select autoretry mode,

connect LTCH/RTR to ABP or leave unconnected.

125mV DOWN =

STOP RAMP THRESHOLD

REFERENCE RAMP

250mV UP =

FAULT THRESHOLD

250mV DOWN =

FAULT THRESHOLD

250mV UP =

FAULT THRESHOLD

250mV DOWN =

FAULT THRESHOLD

125mV UP =

STOP RAMP THRESHOLD

REFERENCE RAMP

POWER-UP

Figure 10. Stop Ramp FAULT Window During Power-Up and Power-Down

POWER-DOWN

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