LTC1649 Linear Technology, LTC1649 Datasheet - Page 10

• Manufacturer Part Number: LTC1649
• Description: 3.3V Input High Power Step-Down Switching Regulator Controller
• Manufacturer: Linear Technology

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