LTC1878 LINER [Linear Technology], LTC1878 Datasheet - Page 10

• Manufacturer Part Number: LTC1878
• Description: High Efficiency Monolithic Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1878
optimized to provide stable high performance transient
response regardless of the output capacitor selected.
ESR is a direct function of the volume of the capacitor.
Manufacturers such as Taiyo-Yuden, AVX, Kemet, Sprague
and Sanyo should be considered for high performance
capacitors. The POSCAP solid electrolytic chip capacitor
available from Sanyo is an excellent choice for output bulk
capacitors due to its low ESR/size ratio. Once the ESR
requirement for C
rating generally far exceeds the I
When using tantalum capacitors, it is critical that they are
surge tested for use in switching power supplies. A good
choice is the AVX TPS series of surface mount tantalum,
available in case heights ranging from 2mm to 4mm. Other
capacitor types include KEMET T510 and T495 series and
Sprague 593D and 595D series. Consult the manufacturer
for other specific recommendations.
Output Voltage Programming
The output voltage is set by a resistive divider according
to the following formula:
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 3.
Phase-Locked Loop and Frequency Synchronization
The LTC1878 has an internal voltage-controlled oscillator
and phase detector comprising a phase-locked loop. This
allows the top MOSFET turn-on to be locked to the rising
edge of an external frequency source. The frequency range
of the voltage-controlled oscillator is 400kHz to 700kHz. The
phase detector used is an edge sensitive digital type that
provides zero degrees phase shift between the
10
V
OUT
Figure 3. Setting the LTC1878 Output Voltage
0 8
.
V
LTC1878
1
OUT
U
R
R
GND
2
V
1
has been met, the RMS current
FB
U
0.8V V
RIPPLE(P-P)
W
OUT
R2
R1
1878 F03
6V
requirement.
U
(2)
external and internal oscillators. This type of phase detec-
tor will not lock up on input frequencies close to the har-
monics of the V
The output of the phase detector is a pair of complemen-
tary current sources charging or discharging the external
filter network on the PLL LPF pin. The relationship
between the voltage on the PLL LPF pin and operating
frequency is shown in Figure 4. A simplified block diagram
is shown in Figure 5.
If the external frequency (V
550kHz, the center frequency, current is sourced
continuously, pulling up the PLL LPF pin. When the
external frequency is less than 550kHz, current is sunk
continuously, pulling down the PLL LPF pin. If the
f
H
is equal to the capture range, f
SYNC/
MODE
Figure 5. Phase-Locked Loop Block Diagram
Figure 4. Relationship Between Oscillator
Frequency and Voltage at PLL LPF Pin
800
700
600
500
400
300
PHASE
DETECTOR
0
FREQUENCY
DETECTOR
DIGITAL
CO
PHASE/
center frequency. The PLL hold-in range
0.4
2.4V
0.8
V
PLL LPF
SYNC/MODE
(V)
1.2
1.6
H
PLL LPF
= f
) is greater than
1878 F04
R
VCO
LP
2.0
C
1878 F05
= 150kHz.
C
LP