ltc3858-2 Linear Technology Corporation, ltc3858-2 Datasheet - Page 24

• Manufacturer Part Number: ltc3858-2
• Description: Ltc3858-2 - Low Iq, Dual 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

LTC3858-2
Topside MOSFET Driver Supply (C
External bootstrap capacitors, C
pins supply the gate drive voltages for the topside MOSFETs.
Capacitor C
external diode D
When one of the topside MOSFETs is turned on, the driver
places the C
MOSFET. This enhances the top MOSFET switch and turns
it on. The switch node voltage, SW, rises to V
BOOST pin follows. With the topside MOSFET on, the
boost voltage is above the input supply: V
V
100 times that of the total input capacitance of the top-
side MOSFET(s). The reverse breakdown of the external
Schottky diode must be greater than V
When adjusting the gate drive level, the final arbiter is the
total input current for the regulator. If a change is made
and the input current decreases, then the efficiency has
improved. If there is no change in input current, then there
is no change in efficiency.
Fault Conditions: Current Limit and Current Foldback
When the output current hits the current limit, the output
voltage begins to drop. If the output voltage falls below
70% of its nominal output level, then the maximum
sense voltage is progressively lowered to about half of
its maximum selected value. Under short-circuit condi-
APPLICATIONS INFORMATION
24
INTVCC
EXTV
1/2 LTC3858-2
. The value of the boost capacitor, C
Figure 9. Capacitive Charge Pump for EXTV
CC
B
B
in the Functional Diagram is charged though
voltage across the gate-source of the desired
PGND
TG1
BG1
SW
V
C
IN
B
IN
MTOP
MBOT
from INTV
V
IN
BAT85
CC
D
B
L
when the SW pin is low.
, connected to the BOOST
VN2222LL
B
R
, D
SENSE
IN(MAX)
B
)
38582 F09
BOOST
B
, needs to be
.
CC
IN
BAT85
BAT85
C
V
OUT
OUT
and the
= V
IN
+
tions with very low duty cycles, the LTC3858-2 will begin
cycle skipping in order to limit the short-circuit current.
In this situation the bottom MOSFET will be dissipating
most of the power but less than in normal operation. The
short-circuit ripple current is determined by the minimum
on-time, t
voltage and inductor value:
The resulting average short-circuit current is:
Phase-Locked Loop and Frequency Synchronization
The LTC3858-2 has an internal phase-locked loop (PLL)
comprised of a phase frequency detector, a lowpass filter,
and a voltage-controlled oscillator (VCO). This allows the
turn-on of the top MOSFET of controller 1 to be locked to
the rising edge of an external clock signal applied to the
PLLIN/MODE pin. The turn-on of controller 2’s top MOSFET
is thus 180 degrees out of phase with the external clock.
The phase detector is an edge sensitive digital type that
provides zero degrees phase shift between the external
and internal oscillators. This type of phase detector does
not exhibit false lock to harmonics of the external clock.
When not prebiased, applying an external clock will invoke
traditional PLL operation. If the external clock frequency is
greater than the internal oscillator’s frequency, f
current is sourced continuously from the phase detector
output, pulling up the VCO input. When the external clock
frequency is less than f
pulling down the VCO input. If the external and internal
frequencies are the same but exhibit a phase difference,
the current sources turn on for an amount of time cor-
responding to the phase difference. The voltage at the
VCO input 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 internal filter capacitor, C
voltage at the VCO input.
I
ΔI
SC
L(SC)
=
50% • I
ON(MIN)
= t
R
ON(MIN)
SENSE
LIM(MAX)
, of the LTC3858-2 (≈95ns), the input
⎛
⎜
⎝
OSC
V
L
IN
–
, current is sunk continuously,
⎞
⎟
⎠
2
1
ΔI
L(SC)
LP
, holds the
OSC
, then
38582f