LTC3789_15 Datasheet, PDF (18/30 Pages) Linear Technology – High Efficiency, Synchronous, 4-Switch Buck-Boost Controller

LTC3789
APPLICATIONS INFORMATION
clock whose frequency is within range of the LTC3789’s
internal VCO. This is guaranteed to be between 200kHz
and 600kHz. A simplified block diagram is shown in
Figure 10.
2.4V 5V
10µA
R_SET
EXTERNAL
OSCILLATOR
MODE/
PLLIN
DIGITAL
PHASE/
FREQUENCY
DETECTOR
SYNC
FREQ
VCO
3789 F10
Figure 10. Phase-Locked Loop Block Diagram
If the external clock frequency is greater than the inter-
nal oscillator’s frequency, f_OSC, then current is sourced
continuously from the phase detector output, pulling up
the filter network. When the external clock frequency is
less than f_OSC, current is sunk continuously, pulling down
the filter network. If the external and internal frequencies
are the same but exhibit a phase difference, the current
sources turn on for the amount of time corresponding to
the phase difference. The voltage on the filter network is
adjusted until the phase and frequency of the internal and
external oscillators are identical. At the stable operating
point, the phase detector output is high impedance and
the filter capacitor holds the voltage.
Typically, the external clock (on the MODE/PLLIN pin)
input high threshold is 1.6V, while the input low thresh-
old is 1V.
Inductor Selection
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. The inductor
value has a direct effect on ripple current. The inductor
current ripple ∆I_L is typically set to 20% to 40% of the
maximum inductor current in the boost region at V_IN(MIN).
For a given ripple the inductance terms in continuous
mode are as follows:
L_BOOST >
V_IN(MIN)2 • (V_OUT – V_IN(MIN)) • 100
f • I_OUT(MAX) • % Ripple • V_OUT2
H,
( ) _{LBUCK >}
V_OUT • V_IN(MAX) – V_OUT • 100
f • I_OUT(MAX) • % Ripple • V_IN(MAX)
H
where:
f is operating frequency, Hz
% Ripple is allowable inductor current ripple
V_IN(MIN) is minimum input voltage, V
V_IN(MAX) is maximum input voltage, V
V_OUT is output voltage, V
I_OUT(MAX) is maximum output load current, A
For high efficiency, choose an inductor with low core
loss, such as ferrite. Also, the inductor should have low
DC resistance to reduce the I²R losses, and must be able
to handle the peak inductor current without saturating. To
minimize radiated noise, use a toroid, pot core or shielded
bobbin inductor.
C_IN and C_OUT Selection
In the boost region, input current is continuous. In the
buck region, input current is discontinuous. In the buck
region, the selection of input capacitor C_IN is driven by
the need to filter the input square wave current. Use a low
ESR capacitor sized to handle the maximum RMS current.
For buck operation, the input RMS current is given by:
I_RMS
≈ I_OUT(MAX)
•
V_OUT
V_IN
•
V_{IN – 1}
V_OUT
This formula has a maximum at V_IN = 2V_OUT, where
I_RMS = I_OUT(MAX)/2. This simple worst-case condition
is commonly used for design because even significant
deviations do not offer much relief. Note that ripple cur-
rent ratings from capacitor manufacturers are often based
on only 2000 hours of life which makes it advisable to
derate the capacitor.
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