LTC3589HUJ#PBF Linear Technology, LTC3589HUJ#PBF Datasheet - Page 22

• 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

LTC3589/LTC3589-1
OPERATION
Table 7. Step-Down Switching Regulator 3 Command Register
Settings
COMMAND
REGISTER[BIT]
SCR1[5-4]
OVEN[2]
SCR2[2]
VCCR[5]
VCCR[4]
VRRCR[5-4]
B3DTV1[5]
B3DTV1[4-0]
B3DTV2[4-0]
B3DTV2[5]
B3DTV2[6]
B3DTV2[7]
* Denotes Default Power-On Value
Operating Frequency
The switching frequency of each of the LTC3589/LTC3589-1
step-down switching regulators may be independently set
using I
and B3DTV2[5]. The power-on default frequency is
2.25MHz. Writing bit BxDTV2[5] HIGH will reduce the
switching frequency to 1.125MHz. Selection of the op-
erating frequency is determined by desired effi ciency,
component size and converter duty cycle.
Operation at lower frequency improves effi ciency by reduc-
ing internal gate charge and switching losses but requires
larger inductance and capacitance values for comparable
output ripple voltage. The lowest duty cycle of the step-
down switching regulator is determined by the converters
minimum on-time. Minimum on-time is the shortest time
duration that the converter is capable of turning its top PMOS
on and off again. The time consists of the gate charge time
22
2
C command register bits B1DTV2[5], B2DTV2[5]
11001*
11001*
VALUE
00*
01
10
00
01
10
11*
0*
1
0*
1
0*
1
1
0*
1
0*
1
0*
1
0*
1
SETTING
Pulse-Skipping Mode
Burst Mode Operation
Forced Continuous Mode
Disable
Enable
Wait for Output Below 300mV Before Enable
Enable Immediately
Select Register B3DTV1 (V1) Reference
Select Register B3DTV2 (V2) Reference
Initiate Dynamic Voltage Slew
Reference Slew Rate = 0.88mV/μs
Reference Slew Rate = 1.75mV/μs
Reference Slew Rate = 3.5mV/μs
Reference Slew Rate = 7mV/μs
Force PGOOD Low When Slewing
Normal PGOOD Operation When Slewing
DAC Dynamic Target Voltage V1
DAC Dynamic Target Voltage V2
2.25MHz Switching Frequency
1.125MHz Switching Frequency
Switch on Clock Phase 1
Switch on Clock Phase 2
Shutdown Regulator 3 Normally
Keep Regulator 3 Alive
plus internal delays associated with peak current sensing.
The minimum on-time of the LTC3589/LTC3589-1 is ap-
proximately 90ns. If the duty cycle falls below what can be
accommodated by the minimum on-time, the converter will
begin to skip cycles. The output voltage will continue to be
regulated but the ripple voltage and current will increase.
With the switching frequency set to 2.25MHz, the minimum
supported duty cycle is 20%. Switching at 1.125MHz the
converter can support a 10% duty cycle.
Phase Selection
To reduce the cycle by cycle peak current drawn by the
switching regulators, the clock phase of each of the LTC3589/
LTC3589-1 step-down switching regulators can be set
using I
and B3DTV2[6]. The internal full-rate clock has a nominal
duty cycle of 20% while the half-rate clocks have a 50%
duty cycle. Setting the command register bits high will
delay the start of each converter switching cycle by 20%
or 50% depending on the selected operating frequency.
Inductor Selection
The choice of step-down switching regulator inductor infl u-
ences the effi ciency of the converter and the magnitude of
the output voltage ripple. Larger inductance values reduce
inductor current ripple and therefore lower output voltage
ripple. A larger value inductor improves effi ciency by low-
ering the peak current to be closer to the average output
current. Larger inductors, however, generally have higher
series resistance that counters the effi ciency advantage
of reduced peak current.
Inductor ripple current is a function of switching frequency,
inductance, V
In an example application the LTC3589/LTC3589-1 step-
down switching regulator 3 has a maximum load of 1A,
V
design point for inductor ripple is 30% of output current
or 300mA. Using the equation for ripple current, a 1.2μH
inductor should be selected.
IN
equals 3.8V, and V
I
L
2
=
C command register bits B1DTV2[6], B2DTV2[6]
f • L
1
IN
• V
, and V
OUT
OUT
1–
OUT
, as shown in this equation:
is set for 1.2V. A good starting
V
V
OUT
IN
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