LTM4609 Datasheet, PDF (10/24 Pages) Linear Technology – 36VIN, 34VOUT High Effi ciency Buck-Boost DC/DC μModule

LTM4609
APPLICATIONS INFORMATION
locked loop comprised of an internal voltage controlled
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-lock 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-lock 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)
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Figure 2. Frequency vs PLLFLTR Pin Voltage
Low Current Operation
To improve the efficiency at low 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. Different Operating Modes
FCB PIN
BUCK
0V to 0.75V Force Continuous Mode
0.85V to 5V
Skip-Cycle Mode
>5.3V
DCM with Constant Freq
BOOST
Force Continuous Mode
Burst Mode Operation
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 0.8V, 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. 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 discon-
tinuous 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:
D=
V_OUT
V_IN
Without considering the inductor current ripple, the RMS
current of the input capacitor can be estimated as:
I_CIN(RMS)
=
I_{OUT(MAX )}
η
•
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 rat-
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