LTC3537 datasheet(13/20 Pages) LINER

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LTC3537

3537fa

OPERATION

Burst Mode operation is inhibited during start-up and soft-

start and until VOUTB is at least 0.24V greater than VINB.

The LTC3537 will operate at a continuous PWM frequency

of 2.2MHz by connecting MODE to GND. At very light loads,

the LTC3537 will exhibit pulse-skip operation.

LDO REGULATOR OPERATION

The LTC3537 includes an independent 100mA low dropout

linear regulator (LDO). The VINL pin can be connected to

an independent source or connected to the output of the

boost regulator. An input capacitor on VINL is optional, but

it will help to improve transient responses. The LDO will

operate with a VINL down to 1.5V, but speciﬁcations are

guaranteed with VINL from 1.8V to 5.5V.

Shutdown

Shutdown of the LDO is accomplished by pulling ENLDO

below 0.3V and enabled by pulling ENLDO above 0.8V. Note

that ENLDO can be driven above VINL or VOLDO, as long

as it is limited to less than the absolute maximum rating.

In the event that the LDO output voltage is held above the

input voltage, the LDO goes in to shutdown until the output

drops back below the input voltage. In shutdown the LDO

will block reverse current from VOLDO to VINL.

LDO Error Ampliﬁer

The non-inverting input of the transconductance error

ampliﬁer is internally connected to a 0.6V reference and

the inverting input is connected to FBL. The control loop

compensation is provided internally. An external resistive

voltage divider from VOLDO to ground programs the output

voltage via FBL from 0.6V to 5V.

OLDO = 0.6V 1+

⎛

⎝⎜

⎞

⎠⎟

LDO Current Sensing and Limiting

Current is sensed across an internal resistor. The guaran-

teed minimum output current is 100mA.

LOW-BATTERY INDICATOR

The LTC3537 includes a low-battery comparator. The non-

inverting input of the comparator is internally connected

to a 0.6V reference and the inverting input is connected

to LBI. An external resistive voltage divider from VINL to

ground programs the threshold voltage. When the volt-

age at LBI drops below 0.6V, the open-drain N-channel

MOSFET will turn on. The N-channel MOSFET device is

forced off when both the step-up converter and LDO are

in shutdown.

LBI = 0.6V 1+

⎛

⎝⎜

⎞

⎠⎟

BOOST POWER-GOOD INDICATOR

The LTC3537 includes a power-good comparator for the

step-up converter. The non-inverting input of the compara-

tor is internally connected to a 1.08V reference and the

inverting input is connected to the FBB pin. The open-drain

MOSFET on PGDB will turn on when the output voltage is

typically within 6% of the programmed output voltage.

Output sequencing can be achieved by connecting PGDB to

the LDO enable pin (ENLDO). This would allow the user to

keep the LDO off until the step-up converter is regulating.

The N-channel MOSFET is forced on in shutdown.

LDO POWER-GOOD INDICATOR

The LTC3537 includes a power-good comparator for the

LDO. The non-inverting input of the comparator is internally

connected to a 540mV reference and the inverting input is

connected to the FBL pin. The open-drain MOSFET on the

PGDL pin will turn on when the output voltage is typically

within 4% of the programmed output voltage.

Output sequencing can be achieved by connecting PGDL to

the boost enable pin (ENBST). This would allow the user to

keep the step-up converter off until the LDO is regulating.

The N-channel MOSFET is forced on in shutdown.

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