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LTM4609
APPLICATIONS INFORMATION
phase-locked loop comprised of an internal voltage con-
trolled 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
load current is lower than the preset minimum output
current level. 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.
0
0.5 1.0 1.5 2.0
PLLFLTR PIN VOLTAGE (V)
2.5
4609 F02
Input Capacitors
V OUT
I CIN(RMS) =
I OUT(MAX)
η
Figure 2. Frequency vs PLLFLTR Pin Voltage
Low Current Operation
To improve efficiency at low output current operation,
LTM4609 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.
Table 2. Differen t Operating Modes (V INTVCC = 6V)
FCB PIN BUCK BOOST
0V to 0.75V Force Continuous Mode Force Continuous Mode
0.85V to Skip-Cycle Mode Burst Mode Operation
V INTVCC – 1V
>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, where V INTVCC is 6V,
the controller enters Burst Mode operation in boost opera-
tion or enters skip-cycle mode in buck operation. During
boost operation, Burst Mode operation is activated if the
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:
D=
V IN
Without considering the inductor current ripple, the RMS
current of the input capacitor can be estimated as:
? D ? (1-D)
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
ceramic capacitors. Note the capacitor ripple current
ratings are often based on temperature and hours of life.
4609ff
10
For more information www.linear.com/LTM4609
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