LTC3425 Datasheet, PDF (11/24 Pages) Linear Technology – 5A, 8MHz, 4-Phase Synchronous Step-Up DC/DC Converter

U
OPERATIO
DETAILED DESCRIPTION
The LTC3425 provides high efficiency, low noise power
for high current boost applications such as cellular phones
and PDAs. The true output disconnect feature eliminates
inrush current and allows V_OUT to go to zero during
shutdown. The current mode architecture with adaptive
slope compensation provides ease of loop compensation
with excellent transient load response. The low R_DS(ON),
low gate charge synchronous switches eliminate the need
for an external Schottky rectifier, and provide efficient high
frequency pulse width modulation (PWM) control. High
efficiency is achieved at light loads when Burst Mode
operation is entered, where the IC’s quiescent current is a
low 12µA typical on V_OUT.
MULTIPHASE OPERATION
The LTC3425 uses a 4-phase architecture, rather than the
conventional single phase of other boost converters. By
having multiple phases equally spaced (90° apart), not
only is the output ripple frequency increased by a factor of
four, but the output capacitor ripple current is greatly
reduced. Although this architecture requires four induc-
tors, rather than a single inductor, there are a number of
important advantages.
• Much lower peak inductor current allows the use of
smaller, lower cost inductors.
• Greatly reduced output ripple current minimizes output
capacitance requirement.
• Higher frequency output ripple is easier to filter for low
noise applications.
• Input ripple current is also reduced for lower noise on
V_IN.
The peak boost inductor current is given by:
I_LPEAK
=
I_O
(1– D)
•N
+
di
2
Where I_O is the average load current, D is the PWM duty
cycle, N is the number of phases and di is the inductor
ripple current. This relationship is shown graphically in
Figure 1 using a single phase and a 4-phase example.
LTC3425
5
SINGLE
PHASE
4
3
FOUR PHASE
2
1
0
0
0.5
1
1.5
TIME (µs)
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Figure 1. Comparison of Output Ripple Current with Single
Phase and 4-Phase Boost Converter in a 2A Load Application
Operating at 50% Duty Cycle
Example:
The following example, operating at 50% duty cycle,
illustrates the advantages of multiphase operation over a
conventional single-phase design.
V_IN = 1.9V, V_OUT = 3.6V, Efficiency = 90% (approx),
I_OUT = 2A, Frequency = 1MHz, L = 2.2µH
Table 1
PARAMETER
SINGLE FOUR CHANGE FROM
PHASE PHASE 1 TO 4 PHASE
Peak-Peak Output
Ripple Current
4.227A 0.450A Reduced by 89%
RMS Output Ripple Current 2.00A 0.184A Reduced by 91%
Peak Inductor Current
4.227A 1.227A Reduced by 71%
Output Ripple Frequency
1MHz 4MHz Increased by 4×
With 4-phase operation, at least one of the phases will be
delivering current to the load whenever V_IN is greater than
one quarter V_OUT (duty cycles less than 75%). For lower
duty cycles, there can be as many as two or three phases
delivering load current simultaneously. This greatly re-
duces both the output ripple current and the peak current
in each inductor, compared with a single-phase converter.
This is illustrated in the waveforms of Figures 2 and 3.
Operation Using Only Two or Three Phases
The LTC3425 can operate as a 2- or 3-phase converter by
simply eliminating the inductor from the unused phase(s).
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