LTC1627 Linear Technology, LTC1627 Datasheet - Page 7

• Manufacturer Part Number: LTC1627
• Description: Monolithic Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

OPERATIO
load slightly allows constant frequency PWM operation
to resume.
Frequency synchronization is inhibited when the feedback
voltage V
from interfering with the frequency foldback for short-
circuit protection.
Dropout Operation
When the input supply voltage decreases toward the out-
put voltage, the duty cycle increases 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 P-channel MOSFET and the inductor.
In Burst Mode operation or pulse skipping mode operation
(externally synchronized) with the output lightly loaded,
the LTC1627 transitions through continuous mode as it
enters dropout.
Undervoltage Lockout
A precision undervoltage lockout shuts down the LTC1627
when V
lithium-ion battery applications. In lockout, the LTC1627
draws only several microamperes, which is low enough to
prevent deep discharge and possible damage to the lithium-
ion battery nearing its end of charge. A 150mV hysteresis
ensures reliable operation with noisy supplies.
Low Supply Operation
The LTC1627 is designed to operate down to 2.65V supply
voltage. At this voltage the converter is most likely to be
running at high duty cycles or in dropout where the main
switch is on continuously. Hence, the I
mainly to the R
Efficiency Considerations in the Applications Information
section.
When V
MOSFET can be lowered by driving its gate below ground.
The top P-channel MOSFET driver makes use of a floating
return pin, V
charge pump bootstrapped to the SW pin realizes a
negative voltage at the V
IN
IN
FB
drops below 2.5V, making it ideal for single
is low (< 4.5V) the R
is below 0.6V. This prevents the external clock
DR
, to allow biasing below GND. A simple
DS(ON)
U
DR
of the P-channel MOSFET. See
pin as shown in Figure 2. Using
DS(ON)
of the P-channel
2
R loss is due
the charge pump at V
ensure that (V
maximum voltage.
When V
may enter dropout and attempt to turn on the P-channel
MOSFET continuously. When the V
enabled, a dropout detector counts the number of oscilla-
tor cycles that the P-channel MOSFET remains on, and
periodically forces a brief off period to allow C1 to
recharge. 100% duty cycle is allowed when V
Slope Compensation and Inductor Peak Current
Slope compensation provides stability by preventing
subharmonic oscillations. It works by internally adding a
ramp to the inductor current signal at duty cycles in excess
of 40%. As a result, the maximum inductor peak current
is lower for V
the inductor peak current as a function of duty cycle graph
in Figure 3. The worst-case peak current reduction occurs
with the oscillator synchronized at its minimum frequency,
i.e., to a clock just above the oscillator free-running
Figure 3. Maximum Inductor Peak Current vs Duty Cycle
LTC1627
IN
Figure 2. Using a Charge Pump to Bias V
decreases to a voltage close to V
950
900
850
800
750
700
650
600
550
500
OUT
V
0
V
SW
DR
IN
IN
V
/V
IN
10 20 30 40 50
– V
= 5V
IN
> 0.4 than when V
DR
IN
C2
0.1 F
WORST CASE
CLOCK SYNC
) does not exceed its absolute
DUTY CYCLE (%)
EXTERNAL
4.5V is not recommended to
D1
D2
L1
60 70 80 90 100
WITHOUT
EXTERNAL
CLOCK SYNC
+
DR
OUT
LTC1627
C
100 F
C1
0.1 F
charge pump is
OUT
1627 F03
DR
/V
OUT
is grounded.
IN
DR
V
V
IN
OUT
< 0.4. See
, the loop
< 4.5V
1627 F02
7