LTC3708_15 Datasheet, PDF (17/32 Pages) Linear Technology – Fast 2-Phase, No RSENSE Buck Controller with Output Tracking

LTC3708
APPLICATIONS INFORMATION
Manufacturers such as Nichicon, United Chemi-Con and
Sanyo can be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest (ESR)(size)
product of any aluminum electrolytic at a somewhat
higher price. An additional ceramic capacitor in parallel
with OS-CON capacitors is recommended to reduce the
inductance effects.
In surface mount applications multiple capacitors may
need to be used in parallel to meet the ESR, RMS current
handling and load step requirements of the application.
Aluminum electrolytic, dry tantalum and special polymer
capacitors are available in surface mount packages. Special
polymer capacitors offer very low ESR but have lower
storage capacity per unit volume than other capacitor
types. These capacitors offer a very cost-effective output
capacitor solution and are an ideal choice when combined
with a controller having high loop bandwidth. Tantalum
capacitors offer the highest capacitance density and are
often used as output capacitors for switching regulators
having controlled soft-start. Several excellent surge-tested
choices are the AVX TPS, AVX TPSV or the KEMET T510
series of surface mount tantalums, available in case heights
ranging from 2mm to 4mm. Aluminum electrolytic capaci-
tors can be used in cost-driven applications providing that
consideration is given to ripple current ratings, temperature
and long term reliability. A typical application will require
several to many aluminum electrolytic capacitors in paral-
lel. A combination of the above mentioned capacitors will
often result in maximizing performance and minimizing
overall cost. Other capacitor types include Nichicon PL
series, Sanyo POSCAP, NEC Neocap, Cornell Dubilier ESRE
and Sprague 595D series. Consult manufacturers for other
specific recommendations.
Top MOSFET Driver Supply
(C_B, D_B in the Functional Diagram)
An external bootstrap capacitor, C_B, connected to the
BOOST pin supplies the gate drive voltage for the topside
MOSFET. This capacitor is charged through diode D_B from
DRV_CC when the switch node is low. Note that the average
voltage across C_B is approximately DRV_CC. When the top
MOSFET turns on, the switch node rises to V_IN and the
BOOST pin rises to approximately V_IN + DRV_CC. The boost
capacitor needs to store about 100 times the gate charge
required by the top MOSFET. In most applications 0.1μF
to 0.47μF is adequate.
Discontinuous Mode Operation and FCB Pin
The FCB pin determines whether the bottom MOSFET
remains on when current reverses in the inductor. Tying
this pin above its 2.3V threshold (typically to V_CC) enables
discontinuous operation where the bottom MOSFET turns
off when inductor current reverses. The load current at
which current reverses and discontinuous operation begins
depends on the amplitude of the inductor ripple current
and the ripple current depends on the choice of inductor
value and operating frequency as well as the input and
output voltages.
Tying the FCB pin below 1.9V forces continuous synchro-
nous operation, allowing current to reverse at light loads
and maintaining high frequency operation.
Besides providing a logic input to force continuous op-
eration, the FCB pin acts as the input for external clock
synchronization. Upon detecting the presence of an ex-
ternal clock signal, channel 1 will lock on to this external
clock and this will be followed by channel 2 (see PLL and
Frequency Synchronization).
The LTC3708 defaults to forced continuous mode when
sychronized to an external clock or when the PGOOD
signal is low.
Fault Conditions: Current Limit
The maximum inductor current is inherently limited in a
current mode controller by the maximum sense voltage.
In the LTC3708, the maximum sense voltage is controlled
by the voltage on the V_RNG pin. With valley current control,
the maximum sense voltage and the sense resistance
determine the maximum allowed inductor valley current.
The corresponding output current limit is:
I_LIMIT
=
V_SNS(MAX)
R_DS(ON) • ρ_T
+
1
2
•
ΔI_L
3708fb
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