LTC3589HUJ#PBF Linear Technology, LTC3589HUJ#PBF Datasheet - Page 20

• Manufacturer Part Number: LTC3589HUJ#PBF
• Description: IC DC/DC CONV 8-OUTPUT 40QFN
• Manufacturer: Linear Technology

Specifications of LTC3589HUJ#PBF

• Applications: Handheld/Mobile Devices
• Current - Supply: 8µA
• Voltage - Supply: 2.7 V ~ 5.5 V
• Operating Temperature: -40°C ~ 150°C
• Mounting Type: Surface Mount
• Package / Case: *
• Primary Input Voltage: 5.5V
• No. Of Outputs: 8
• Output Voltage: 5V
• Output Current: 1.6A
• No. Of Pins: 40
• Operating Temperature Range: -40°C To +150°C
• Msl: MSL 1 - Unlimited
• No. Of Ldo Regulators: 3
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

OPERATION
LTC3589/LTC3589-1
A value of 10pF is recommended for most applications.
Experimentation with capacitor sizes between 10pF and
33pF may yield improved transient response.
Operating Modes
The step-down switching regulators include three possible
operating modes to meet the noise and power needs of a
variety of applications.
In pulse-skipping mode, at the start of every cycle, a latch
is set that turns on the main P-channel MOSFET switch.
During the cycle, a current comparator compares the
peak inductor current to the output of an error amplifi er.
The output of the current comparator resets the latch.
At this time the P-channel MOSFET switch turns off and
the N-channel MOSFET synchronous rectifi er turns on.
The N-channel MOSFET synchronous rectifi er will turn
off when the end of the clock cycle is reached or if the
inductor current drops through zero. Using this method
of operation, the error amplifi er adjusts the peak inductor
current to deliver the required output power. All necessary
loop compensation is internal to the step-down switching
regulator requiring only a single ceramic output capacitor
for stability. At light loads in pulse-skipping mode, the
inductor current may reach zero on each pulse that will
turn off the N-channel MOSFET synchronous rectifi er. In
this case the switch node (SW1, SW2, or SW3) goes HIGH
impedance and the switch node will ring. This is discon-
tinuous operation and is normal behavior for a switching
regulator. At very light loads in pulse-skipping mode, the
step-down switching regulators will automatically skip
pulses as needed to maintain output regulation. At high
duty cycle (V
current to reverse at light loads causing the step-down
switching regulator to operate continuously. When operat-
ing continuously, regulation and low noise output voltage
are maintained, but input operating current will increase
to a few milliamps.
In the forced continuous mode of operation, the inductor
current is allowed to be less than zero over the full range
of duty cycles. Operating in forced continuous mode is
a lower noise option at light loads than pulse-skipping
operation but with the drawback of higher V
20
OUTX
> V
IN
/2) it is possible for the inductor
IN
current
due to the continuous operation of the MOSFET switch
and rectifi er. Since the inductor current is allowed to be
negative in forced continuous operation the step-down
switching regulator has the ability to sink output current.
The LTC3589/LTC3589-1 automatically forces the step-
down switching regulator into forced continuous mode
when dynamically slewing the DAC voltage reference down.
When the LTC3589/LTC3589-1 step-down switching regula-
tors are in Burst Mode operation, they automatically switch
between fi xed frequency pulse-skipping operation and hys-
teretic Burst Mode control as a function of the load current.
At light loads the step-down switching regulators control
the inductor current directly and use a hysteretic control
loop to minimize both noise and switching losses. While in
Burst Mode operation, the output capacitor is charged to a
voltage slightly higher than the regulation point. The step-
down switching regulator then goes into a low power sleep
mode during which the output capacitor provides the load
current. In sleep mode, most of the switching regulator’s
circuitry is powered off to conserve battery power. When
the output voltage drops below the regulation point the
regulator’s circuitry is powered on and another burst cycle
begins. As the load current increases, the time between burst
cycles decreases. Above a load current about one-quarter
rated output load, the step-down switching regulators will
switch to low noise constant-frequency PWM operation.
Set the mode of operation for the step-down switching
regulators by using the I
of the three regulators has independent mode control.
A step-down switching regulator may enter a dropout condi-
tion when its input voltage drops to near its programmed
output voltage. For example, a discharging battery voltage
of 3.4V dropping to the regulators programmed output
voltage of 3.3V. When this happens the duty cycle of the
P-channel MOSFET switch is increased until it turns on
continuously with 100% duty cycle. In dropout, the regu-
lators output voltage equals the regulators input voltage
minus the voltage drops across the internal P-channel
MOSFET and the inductor DC resistance.
Table 5, Table 6, and Table 7 show the I
settings used to control the step-down switching regulators.
2
C command register SCR1. Each
2
C command register
3589fc