Datasheet

10
LTC1649
tors that work well in LTC1649 applications. A common
way to lower ESR and raise ripple current capability is to
parallel several capacitors. A typical LTC1649 application
might require an input capacitor with a 5A ripple current
capacity and 2% output shift with a 10A output load step,
which requires a 0.005Ω output capacitor ESR. Sanyo OS-
CON part number 10SA220M (220µF/10V) capacitors
feature 2.3A allowable ripple current at 85°C and 0.035Ω
ESR; three in parallel at the input and seven at the output
will meet the above requirements.
Input Supply Considerations/Charge Pump
The LTC1649 requires four supply voltages to operate:
V
IN
, V
CC
, PV
CC1
and PV
CC2
. V
IN
is the primary high power
input, supplying current to the drain of Q1 and the input to
the internal charge pump at the V
IN
pin. This supply must
be between 2.7V and 6V for the LTC1649 to operate
properly. An internal charge pump uses the voltage at V
IN
to generate a regulated 5V output at CP
OUT
. This charge
pump requires an external 1µF capacitor connected be-
tween the C
+
and C
–
pins, and an external 10µF reservoir
capacitor connected from CP
OUT
to ground. The voltage at
CP
OUT
must always be greater than or equal to V
IN
. If V
IN
is expected to rise above 5V, an additional Schottky diode
(D5) should be added from V
IN
to CP
OUT
.
CP
OUT
is typically connected to PV
CC2
directly, providing
the 5V supply that the G2 driver output uses to drive Q2.
PV
CC2
requires a 10µF bypass to ground; this capacitor
can double as the CP
OUT
reservoir capacitor, allowing a
typical application with CP
OUT
and PV
CC2
connected to-
gether to get away with only a single 10µF capacitor at this
node, located close to the PV
CC2
pin. V
CC
can also be
powered from CP
OUT
, but is somewhat sensitive to noise.
PV
CC2
happens to be a significant noisemaker, so most
applications require an RC filter from CP
OUT
/PV
CC2
to V
CC
.
22Ω and 10µF are typical filter values that work well in
most applications.
PV
CC1
needs to be boosted to a level higher than CP
OUT
to
provide gate drive to Q1. The LTC1649 initially used a
charge pump from V
IN
to create CP
OUT
; the typical appli-
cation uses a second charge pump to generate the PV
CC1
supply. This second charge pump consists of a Schottky
diode (D
CP
) from CP
OUT
to PV
CC1
, and a 1µF capacitor
from PV
CC1
to the source of Q1. While Q2 is on, the diode
charges the capacitor to CP
OUT
. When Q1 comes on, its
source rises to V
IN
, and the cap hauls PV
CC1
up to (CP
OUT
+ V
IN
), adequate to fully turn on Q1. When Q1 turns back
off, PV
CC1
drops back down to CP
OUT
; fortunately, we’re
not interested in turning Q1 on at this point, so the lower
voltage doesn’t cause problems. The next time Q1 comes
on, PV
CC1
bounces back up to (CP
OUT
+ V
IN
), keeping Q1
happy. Figure 4 shows a complete power supply circuit for
the LTC1649.
Figure 4. LTC1649 Power Supplies
APPLICATIONS INFORMATION
WUU
U
DRIVE
CIRCUITRY
V
CC
CHARGE
PUMP
V
IN
LTC1649
CP
OUT
PV
CC2
10µF
PV
CC1
Q1
L1
Q2
G1
G2
V
IN
C
OUT
V
OUT
1649 F04
+
C
IN
+
+
10µF
*OPTIONAL
FOR V
IN
≥ 5V
22Ω
+
1µF
1µF
C
+
C
–
D
CP
D5*