LTC3124 datasheet(11/36 Pages) AD

11 / 36 page

LT3120

Rev. 0

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PIN FUNCTIONS

BST3 (Pin 1): Flying Capacitor Pin for Output Current

Sense Amplifier. This pin must be connected to PVOUT

through a 22nF capacitor.

PVOUT (Pins 2, 4): Output Voltage Power Connection.

These pins are connected to switch D of the buck-boost

converter. Connect a low ESR capacitor between these

pins and GND using the lowest impedance path possible.

GND (Pins 3, 11, 18, 22, Exposed Pad Pin 29): Ground

Connection. These pins should be connected to the board

ground using the shortest and widest connections pos-

sible. High via density should be used under the exposed

pad to maximize heat transfer away from the part.

ISP (Pin 5): Current Sense Positive Input. Connect this pin

to the IC side of the output current sense resistor. If cur-

rent sense is not being used, connect this pin to PVOUT.

ISN (Pin 6): Current Sense Negative Input. Connect this

pin to the VOUT side of the output current sense resistor. If

current sense is not being used, connect this pin to PVOUT.

PGOOD (Pin 7): Open Drain Output Indicator. When FB

drops too far below its regulated voltage this output pulls to

ground. Connect a pull-up resistor from this pin to a posi-

tive supply. Refer to the Operation section for more detail.

PROG (Pin 8): Output Average Current Limit Set Point. A

resistor should be connected between this pin and GND

to program the maximum average output current. The

output current from this pin can also be used as an analog

output current indicator. To disable this function, the pin

should be connected to VCC.

FB (Pin 9): Feedback Voltage Input. A resistive divider

connected to this pin sets the output voltage for the buck-

boost converter. The nominal FB voltage is 0.795V.

VC (Pin 10): Error Amplifier Output. A frequency compen-

sation network must be connected between this pin and

GND to stabilize the voltage control loop.

MPPC (Pin 12): Maximum Power Point Control Pin

Setpoint. Connect this pin to a resistive divider from VIN

to GND to set the input regulation voltage. When not being

used, the MPPC pin should be tied to VCC.

VCC (Pin 13): Internal Regulator Output Voltage. This pin

is the output of the internal low voltage linear regulator

used to supply the control circuitry. A 4.7µF capacitor

should be connected between this pin and GND using the

shortest trace possible. An additional external load of up

to 10mA may be drawn from this output.

RT (Pin 14): Oscillator Frequency Programming Pin.

Connect a resistor between this pin and GND to set the

buck-boost converter switching frequency.

SYNC/MODE (Pin 15): Automatic Burst Mode Operation/

PWM Mode Control Pin and Synchronization Input. Forcing

this pin high causes the IC to operate in fixed frequency

PWM mode at all loads using the internal oscillator at the

frequency set by the RT Pin. Forcing this pin low, causes the

IC to enable Burst Mode operation at light loads to maximize

efficiency. Clocking this pin will cause the part to synchronize

to the clock for frequencies higher than the frequency pro-

grammed by the RT Pin. When using this pin for synchroni-

zation, a minimum input pulse width of 100ns must be used.

EN/UVLO (Pin 16): Input to Enable and Disable the IC and

Set Custom Input UVLO Threshold. The EN/UVLO pin can

be driven by external logic signals to enable and disable the

IC. In addition, the voltage on this pin can be set by a resis-

tive divider connected to the input voltage to provide an

accurate undervoltage lockout threshold. The IC is enabled

if the EN/UVLO pin voltage exceeds 1.215V nominally.

VIN (Pin 17): Input Voltage Pin for Internal VCC Regulator.

PVIN (Pins 19, 20, 21): Input Voltage Power Connection.

These pins are connected to switch A of the buck-boost

converter. Connect a 10µF or larger capacitor between these

pins and GND using the lowest impedance path possible.

BST1 (Pin 23): Flying Capacitor Pin for SW1. This pin must

be connected to SW1 through a 0.22µF capacitor. This pin

is used to generate the gate drive rail for power switch A.

SW1 (Pins 24, 25): Buck-Boost Converter Power Switch

Pin. These pins should be connected to one side of the

buck-boost inductor.

SW2 (Pins 26, 27): Buck-Boost Converter Power Switch

Pin. These pins should be connected to one side of the

buck-boost inductor.

BST2 (Pin 28): Flying Capacitor Pin for SW2. This pin

must be connected to SW2 through a 0.22µF capacitor.

This pin is used to generate the gate drive rail for power

switch D. The capacitance on this node should be 10

times greater than that used between BST3 and PVOUT.

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