LT8650SPJVPBF datasheet(25/34 Pages) AD

25 / 34 page

LT8650SP

Rev. B

For more information www.analog.com

APPLICATIONS INFORMATION

Clock Output

The CLKOUT pin outputs a clock which can be used to

synchronous other regulators to the LT8650SP. In Burst

Mode (SYNC pin low), the CLKOUT pin is grounded. In

forced continuous mode (SYNC pin float or DC high), the

CLKOUT pin outputs a 50% duty cycle clock where the

CLKOUT rising edge is approximately 90 degrees phase

shifted relative to channel 1. If this CLKOUT waveform is

applied to the SYNC pin of another LT8650SP regulator,

then four-phase operation can be achieved. If an external

clock is applied to the SYNC pin of the LT8650SP, then the

CLKOUT pin will output a waveform with the same phasing

and duty cycle as the SYNC pin clock. The low and high

levels of the CLKOUT pin are ground and VCC,respectively.

The drive strength of the CLKOUT pin is several hundred

ohms, so the CLKOUT waveform has rise and fall times

of several tens of ns. The edge rates will be slower if the

CLKOUT trace has extra capacitance.

Temperature Monitor Function

The TEMP pin will output a voltage proportional to die

temperature. The TEMP pin typically outputs 250mV for

25°C and has a slope of 9.5mV/°C. Without the aid of an

external circuitry, the TEMP pin output is valid from 20°C

to 150°C (200mV to 1.5V). Do not load the TEMP pin

with more than 100µA. To extend the TEMP pin output

below 20°C, connect a resistor from the TEMP pin to a

negative voltage.

The TEMP pin can report a higher than actual temperature

at high load currents on channel 1, particularly at high

temperature, high VIN, and high inductor current ripple.

For this reason, the TEMP pin should not be used for ac-

tive monitoring or servo loops.

As a safeguard, the LT8650SP has an additional thermal

shutdown set at a typical value of 165°C. This is an in-

dependent circuit from the TEMP pin monitor circuit If

the thermal shutdown is exceeded, both channels of the

LT8650SP will be shutdown until the thermal overload

event expires.

It should be noted that the TEMP pin voltage represents

the steady-state, average die temperature and should not

beusedtoguaranteethatmaximumjunctiontemperatures

are not exceeded. Instantaneous power along with ther-

mal gradients and time constants may cause portions of

the die to exceed maximum ratings. Be sure to calculate

die temperature rise for steady state (>1 Min) as well as

impulse conditions.

Shorted and Reversed Input Protection

The LT8650SP will tolerate a shorted output. The bottom

switch current is monitored such that if inductor current

is beyond safe levels switching of the top switch will be

delayed until such time as the inductor current falls to

safe levels. Fault condition of one channel will not affect

the operation of the other channel, unless the part goes

into thermal shutdown.

Thereisanothersituationtoconsiderinsystemswherethe

output will be held high when the input to the LT8650SP is

absent. This may occur in battery charging applications or

in battery backup systems where a battery or some other

supply is ORed with channel 1’s output. If the VIN1 pin is

allowed to float and either EN/UV pin is held high (either

by a logic signal or because it is tied to VIN1), then the

LT8650SP’sinternalcircuitrywillpullitsquiescentcurrent

through its SW1 pin. This is acceptable if the system can

tolerate current draw in this state. If both EN/UV pins are

grounded the SW1 pin current will drop to near 1.7µA.

However, if the VIN1pinisgroundedwhilechannel1output

is held high, regardless of EN/UV1, parasitic body diodes

inside the LT8650SP can pull current from the output

through the SW1 pin and the VIN1 pin, damaging the IC.

VIN2 is not connected to the shared internal supply and

will not draw any current if left floating. If both VIN1 and

VIN2 are floating, regardless of EN/UV pins states, no load

will be present at the output of channel 2. However, if the

VIN2 pin is grounded while channel 2 output is held high,

parasitic body diodes inside the LT8650SP can pull cur-

rent from the output through the SW2 pin and the VIN2

pin, damaging the IC.

Figure 8 shows a connection of the VIN and EN/UV pins

that will allow the LT8650SP to run only when the input

voltage is present and that protects against a shorted or

reversed input.

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