LTC1879 LINER [Linear Technology], LTC1879 Datasheet - Page 9

• Manufacturer Part Number: LTC1879
• Description: 1.2A Synchronous Step-Down Regulator with 15mA Quiescent Current
• Manufacturer: LINER [Linear Technology]

OPERATIO
Power Good Indicator
The power good function monitors the output voltage in all
modes of operation. Its open-drain output is pulled low
when the output voltage is not within 7.5% of its nomi-
nally regulated voltage. The feedback voltage is filtered
before it is fed to a power good window comparator in
order to prevent false tripping of the power good signal
during fast transients. The window comparator monitors
the output voltage even in Burst Mode operation. In
shutdown mode, open drain is actively pulled low to
indicate that the output voltage is invalid.
Short-Circuit Protection
When the output is shorted to ground, the frequency of
the oscillator is reduced to about 80kHz, 1/7 the nominal
frequency. This frequency foldback ensures that the in-
ductor current has more time to decay, thereby preventing
runaway. The oscillator’s frequency will progressively
increase to 550kHz (or to the synchronized frequency)
when V
Frequency Synchronization
The LTC1879 can be synchronized to an external clock
source connected to the SYNC/MODE pin. The turn-on of
the top MOSFET is synchronized to the rising edge of the
external clock.
When the LTC1879 is clocked by an external source, Burst
Mode operation is disabled. In this synchronized mode,
when the output load current is very low, current compara-
tor, I
the main switch to stay off for the same number of cycles.
Increasing the output load slightly allows constant fre-
quency PWM operation to resume.
Frequency synchronization is inhibited when the feedback
voltage V
from interfering with the frequency foldback for short-
circuit protection.
COMP
FB
FB
, may remain tripped for several cycles and force
rises above 0.3V.
is below 0.6V. This prevents the external clock
U
(Refer to Block Diagram)
Low Dropout Operation
When the input supply voltage decreases toward the
output voltage in a buck regulator, the duty cycle in-
creases toward the maximum on-time. Further reduction
of the supply voltage forces the main switch to remain on
for more than one cycle until it reaches 100% duty cycle.
The output voltage will then be determined by the input
voltage minus the voltage drop across the top MOSFET
and the inductor.
Low Supply Operation
The LTC1879 is designed to operate down to an input
supply voltage of 2.65V although the maximum allowable
output current is reduced at this low voltage. Figure 2
shows the reduction in the maximum output current as a
function of input voltage.
Another important detail to remember is that at low input
supply voltages, the R
increases. Therefore, the user should calculate the power
dissipation when the LTC1879 is used at 100% duty cycle
with low supply voltage (see Thermal Considerations in
the Applications Information section).
Figure 2. Maximum Output Current vs Input Voltage
1800
1600
1200
1000
1400
400
800
600
2
3
V
OUT
V
4
OUT
= 1.8V
INPUT VOLTAGE (V)
= 2.5V
DS(ON)
5
V
6
OUT
= 3.1V
of the P-channel switch
7
8
9
1879 F02
LTC1879
10
9
1879f