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LTM4607
APPLICATIONS INFORMATION
oscillator and a phase detector. This allows turning on the
internal top MOSFET for locking to the rising edge of the
external clock. A pulse detection circuit is used to detect a
clock on the PLLIN pin to turn on the phase-locked loop.
The input pulse width of the clock has to be at least 400ns,
and 2V in amplitude. The synchronized frequency ranges
from 200kHz to 400kHz, corresponding to a DC voltage
input from 0V to 2.4V at PLLFLTR. During the start up of
the regulator, the phase-locked loop function is disabled.
450
400
350
300
250
200
150
100
50
0
0 0.5 1.0 1.5 2.0 2.5
PLLFLTR PIN VOLTAGE (V)
4607 F02
Figure 2. Frequency vs PLLFLTR Pin Voltage
Low Current Operation
To improve efficiency at low output current operation,
LTM4607 provides three modes for both buck and boost
operations by accepting a logic input on the FCB pin. Table
2 shows the different operation modes.
The MOSFETs will turn on for several cycles, followed by a
variable “sleep” interval depending upon the load current.
During buck operation, skip-cycle mode sets a minimum
positive inductor current level. In this mode, some cycles
will be skipped when the output load current drops below
1% of the maximum designed load in order to maintain
the output voltage.
When the FCB pin voltage is tied to the INTV CC pin, the
controller enters constant frequency discontinuous current
mode (DCM). For boost operation, if the output voltage is
high enough, the controller can enter the continuous current
buck mode for one cycle to discharge inductor current.
In the following cycle, the controller will resume DCM
boost operation. For buck operation, constant frequency
discontinuous current mode is turned on if the preset
minimum negative inductor current level is reached. At
very light loads, this constant frequency operation is not
as efficient as Burst Mode operation or skip-cycle, but
does provide low noise, constant frequency operation.
Input Capacitors
In boost mode, since the input current is continuous, only
minimum input capacitors are required. However, the input
current is discontinuous in buck mode. So the selection
of input capacitor C IN is driven by the need of filtering the
input square wave current.
For a buck converter, the switching duty-cycle can be
estimated as:
Table 2. Different Operating Modes
FCB PIN BUCK
BOOST
D =
V OUT
V IN
I CIN(RMS) =
I OUT(MAX)
0Vto0.75V       ForceContinuousMode       ForceContinuousMode
0.85V to 5V Skip-Cycle Mode Burst Mode Operation
>5.3V DCM with Constant Freq DCM with Constant Freq
When the FCB pin voltage is lower than 0.8V, the controller
behaves as a continuous, PWM current mode synchronous
switching regulator. When the FCB pin voltage is below
V INTVCC – 1V, but greater than 0.85V, the controller enters
Burst Mode operation in boost operation or enters skip-
cycle mode in buck operation. During boost operation,
Burst Mode operation is activated if the load current is
lower than the preset minimum output current level.
Without considering the inductor current ripple, the RMS
current of the input capacitor can be estimated as:
? D ? (1 ? D)
η
In the above equation, η is the estimated efficiency of the
power module. C IN can be a switcher-rated electrolytic
aluminum capacitor, OS-CON capacitor or high volume cer-
amic capacitors. Note the capacitor ripple current ratings
are often based on temperature and hours of life. This
makes it advisable to properly derate the input capacitor,
10
For more information www.linear.com/LTM4607
4607fc
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