LTC3857IUH#TRPBF Linear Technology, LTC3857IUH#TRPBF Datasheet - Page 14

• Manufacturer Part Number: LTC3857IUH#TRPBF
• Description: IC CTRLR STP-DN SYNC DUAL 32QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3857IUH#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 24 V
• Frequency - Switching: 50kHz ~ 900kHz
• Voltage - Input: 4 ~ 38 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -

OPERATION
pulses increased the total RMS current flowing from the
input capacitor, requiring the use of more expensive input
capacitors and increasing both EMI and losses in the input
capacitor and battery.
With 2-phase operation, the two channels of the dual
switching regulator are operated 180 degrees out of phase.
This effectively interleaves the current pulses drawn by the
switches, greatly reducing the overlap time where they add
together. The result is a significant reduction in total RMS
input current, which in turn allows less expensive input
capacitors to be used, reduces shielding requirements for
EMI and improves real world operating efficiency.
Figure 1 compares the input waveforms for a single-phase
dual switching regulator to a 2-phase dual switching
regulator. An actual measurement of the RMS input cur-
rent under these conditions shows that 2-phase operation
dropped the input current from 2.53A
While this is an impressive reduction in itself, remember
that the power losses are proportional to I
that the actual power wasted is reduced by a factor of 2.66.
The reduced input ripple voltage also means less power is
lost in the input power path, which could include batter-
ies, switches, trace/connector resistances and protection
circuitry. Improvements in both conducted and radiated
EMI also directly accrue as a result of the reduced RMS
input current and voltage.
LTC3857
14
Figure 1. Input Waveforms Comparing Single-Phase (a) and 2-Phase (b) Operation for Dual Switching Regulators
Converting 12V to 5V and 3.3V at 3A Each. The Reduced Input Ripple with the 2-Phase Regulator Allows
Less Expensive Input Capacitors, Reduces Shielding Requirements for EMI and Improves Efficiency
(Refer to the Functional Diagram)
I
IN(MEAS)
= 2.53A
RMS
RMS
RMS
to 1.55A
2
, meaning
RMS
INPUT CURRENT
INPUT VOLTAGE
3.3V SWITCH
5V SWITCH
500mV/DIV
20V/DIV
20V/DIV
.
5A/DIV
Of course, the improvement afforded by 2-phase operation
is a function of the dual switching regulator’s relative duty
cycles which, in turn, are dependent upon the input voltage
V
input current varies for single-phase and 2-phase operation
for 3.3V and 5V regulators over a wide input voltage range.
It can readily be seen that the advantages of 2-phase op-
eration are not just limited to a narrow operating range,
for most applications is that 2-phase operation will reduce
the input capacitor requirement to that for just one channel
operating at maximum current and 50% duty cycle.
IN
(Duty Cycle = V
3.0
2.5
2.0
1.5
1.0
0.5
Figure 2. RMS Input Current Comparison
0
0
I
IN(MEAS)
V
V
O1
O2
= 5V/3A
= 3.3V/3A
OUT
10
= 1.55A
/V
INPUT VOLTAGE (V)
DUAL CONTROLLER
IN
RMS
). Figure 2 shows how the RMS
SINGLE PHASE
DUAL CONTROLLER
2-PHASE
20
3857 F01
30
3857 F02
40
3857fc