LTC1433 Datasheet, PDF (12/20 Pages) Linear Technology – 450mA, Low Noise Current Mode Step-Down DC/DC Converters

LTC1433/LTC1434
APPLICATIONS INFORMATION
1.3f_O
f_O
0.7f_O
0
0.5
1.0
1.5
2.0
2.5
V_PLLLPF (V)
1433/34 F09
Figure 9. Relationship Between Oscillator Frequency
and Voltage at PLL LPF Pin
EXTERNAL
FREQUENCY
2.4V
PHASE
DETECTOR
R_LP
C_LP
C_OSC
PLLIN
PLL LPF
C_OSC
DIGITAL
PHASE/
FREQUENCY
OSC
50k
DETECTOR
1433/34 F10
Figure 10. Phase-Locked Loop Block Diagram
If the external frequency (V_PLLIN) is greater than the center
frequency f₀, current is sourced continuously, pulling up
the PLL LPF pin. When the external frequency is less than
f₀, current is sunk continuously, pulling down the PLL LPF
pin. If the external and internal frequencies are the same
but exhibit a phase difference, the current sources turn on
for an amount of time corresponding to the phase differ-
ence. Thus the voltage on the PLL LPF pin is adjusted until
the phase and frequency of the external and internal
oscillators are identical. At this stable operating point the
phase comparator output is open and the filter capacitor
C_LP holds the voltage.
The loop filter components C_LP and R_LP smooth out the
current pulses from the phase detector and provide a
stable input to the voltage controlled oscillator. The filter
components C_LP and R_LP determine how fast the loop
acquires lock. Typically R_LP = 10k and C_LP is 0.01µF to
0.1µF. Be sure to connect the low side of the filter to SGND.
The PLL LPF pin can be driven with external logic to obtain
a 1:1.9 frequency shift. The circuit shown in Figure 11 will
provide a frequency shift from f_O to 1.9f_O as the voltage
V_{PLL LPF} increases from 0V to 2.4V. Do not exceed 2.4V on
V_{PLL LPF}.
PLL LPF
2.4V MAX
3.3V OR 5V
18k
1433/34 F11
Figure 11. Directly Driving PLL LPF Pin
Low-Battery Comparator
The LTC1433/LTC1434 have an on-chip, low-battery com-
parator which can be used to sense a low-battery condi-
tion when implemented as shown in Figure 12. The resis-
tor divider R3/R4 sets the comparator trip point as follows:
V_LBTRIP
=
1.19


R4
R3

+ 1_
V_IN
R4
R3
LTC1433/LTC1434
–
+
1.19V REFERENCE
1433/34 F12
Figure 12. Low-Battery Comparator
The divided down voltage at the negative (–) input to the
comparator is compared to an internal 1.19V reference. A
40mV hysteresis is built in to assure rapid switching. The
output is an open-drain MOSFET and requires a pull-up
resistor to operate. This comparator is active in shutdown.
To save more shutdown quiescent current, this compara-
tor can be shut down by taking the LBI pin below 0.74V,
12