LT3504
9
3504fa
For more information www.linear.com/LT3504
operaTion
A comparator starts the reference when the EN/UVLO pin
rises above the 1.44V rising threshold. Other comparators
prevent switching when the input voltage is below 2.9V or
the die temperature is above 175°C. When the EN/UVLO
is above 1.44V, the input voltage is above 3.2V, and the
temperature is below 175°C, the boost regulator begins
switching and charges the SKY capacitor to 4.85V above
V
IN
. When the SKY voltage is less than 4.5V above V
IN
,
the RUN/SS pins and V
C
nodes are actively pulled low to
prevent the buck regulators from switching.
The boost regulator (Channel 5) consists of an internal
0.4A power switch (Q5), an internal power Schottky diode
(D5), and the necessary logic and other control circuitry
to drive the switch. The switch current is monitored to
enforce cycle-by-cycle current limit. The diode current
is monitored to prevent inductor current runaway during
transient conditions. An error amplifier servos the SKY
voltage to 4.85V above V
IN
. A comparator detects when
the SKY voltage is 4.5V above V
IN
and allows the buck
regulators to begin switching.
The oscillator produces two antiphase clock signals running
at 50% duty cycle. Channels 1, 3 and 5 run antiphase to
Channels 2 and 4. The oscillator can be programmed by
connecting a single resistor from RT/SYNC to ground, or
by applying an external clock signal to RT/SYNC. A sync
detect circuit distinguishes between the type of input.
Tying a resistor to GND directly sets the bias current of
the oscillator. The sync signal is converted to a current to
set the bias current of the oscillator.
The oscillator enables an R
S
flip-flop, turning on the
internal 1.7A power switch Q1. An amplifier and compara-
tor monitor the current flowing between the V
IN
and SW
pins, turning the switch off when this current reaches a
level determined by the voltage at the V
C
node. A second
comparator enforces a catch diode current limit to prevent
inductor current runaway during transient conditions. An
error amplifier measures the output voltage through an
external resistor tied to the FB pin and servos the V
C
node.
If the error amplifier’s output increases, more current is
delivered to the output; if it decreases, less current is
delivered. A clamp on the V
C
pin provides switch current
limit. Each buck regulator switch driver operates by drawing
current from the SKY pin. Regulating the SKY pin to 4.85V
above the V
IN
pin voltage is necessary to fully saturate the
bipolar power switch for efficient operation.
Soft-start is implemented by generating a voltage ramp at
the RUN/SS pin. An internal 1.3µA current source pulls the
RUN/SS pin up to 2.1V. Connecting a capacitor from the
RUN/SS pin to ground programs the rate of the voltage
ramp on the RUN/SS pin. A voltage follower circuit with a
0.1V offset connected from the RUN/SS pin to the RAMP
node prevents switching until the voltage at the RUN/SS
pin increases above 0.1V. When the voltage at the RAMP
node is less than 0.9V, the error amplifier servos the FB
voltage to the RAMP node voltage. When the RAMP node
voltage increases above 0.9V, then the error amplifier ser
-
vos the FB voltage to 0.8V. Additionally, a current amplifier
reduces the catch diode current limit when the FB voltage
is below 0.8V to limit the inductor current during startup.
Each individual buck regulator can be placed in shutdown
by pulling the respective RUN/SS pin below 0.1V. The EN/
UVLO pin can be pulled low (below a V
BE
) to place the
entire part in shutdown, disconnecting the outputs and
reducing the input current to less than 2µA.
The LT3504 is pin compatible with the 28-lead QFN package
LT3514. The LT3514 is a three channel step-down converter
and has one channel (CH3) that outputs 2A instead of 1A.
