lt3688fe Linear Technology Corporation, lt3688fe Datasheet - Page 18

• Manufacturer Part Number: lt3688fe
• Description: Lt3688 - Dual 800ma Step-down Switching Regulator With Power-on Reset And Watchdog Timer
• Manufacturer: Linear Technology Corporation

LT3688
APPLICATIONS INFORMATION
Low Ripple Burst Mode Operation
To enhance effi ciency at light loads, the LT3688 operates
in low ripple Burst Mode operation that keeps the output
capacitor charged to the proper voltage while minimizing
the input quiescent current. During Burst Mode opera-
tion, the LT3688 delivers single cycle bursts of current
to the output capacitor followed by sleep periods where
the output power is delivered to the load by the output
capacitor. Because the LT3688 delivers power to the
output with single, low current pulses, the output ripple
is kept below 25mV for a typical application. In addition,
V
65μA and 155μA, respectively, during the sleep time. As
the load current decreases towards a no-load condition,
the percentage of time that the LT3688 operates in sleep
mode increases and the average input current is greatly
reduced, resulting in high effi ciency even at very low loads
(see Figure 5). At higher output loads the LT3688 will be
running at the frequency programmed by the R
and will be operating in standard PWM mode. The transi-
tion between PWM and low ripple Burst Mode operation
is seamless, and will not disturb the output voltage. The
front page application circuit will switch at full frequency
at output loads higher than about 60mA.
BST and BIAS Pin Considerations
Capacitor C3 and the internal boost Schottky diodes (see
the Block Diagram) are used to generate boost voltages
that are higher than the input voltage. In most cases, a
0.22μF capacitor will work well. For the best performance
in dropout, use a 1μF or larger capacitor. Figure 6 shows
18
IN
and BIAS quiescent currents are reduced to typically
10mV/DIV
0.2A/DIV
5V/DIV
V
V
OUT
SW
Figure 5. Burst Mode Operation
I
L
5μs/DIV
3688 F05
T
resistor,
three ways to arrange the boost circuit. The BST pin must
be more than 2.3V above the SW pin for best effi ciency.
For outputs of 3V and above, the standard circuit (Figure
6a) is best. For outputs between 2.8V and 3V, use a 1μF
boost capacitor. A 2.5V output presents a special case
because it is marginally adequate to support the boosted
drive stage while using the internal boost diode. For reliable
BST pin operation with 2.5V outputs, use a good external
Schottky diode (such as the ON semi MBR0540), and a
1μF boost capacitor (see Figure 6b). For lower output
voltages, the boost diode can be tied to the input (Figure
6c), or to another supply greater than 2.8V. The circuit in
Figure 6a is more effi cient because the BST pin current
and BIAS pin quiescent current comes from a lower volt-
Figure 6. Three Circuits for Generating the Boost Voltage
4.7μF
4.7μF
4.7μF
V
V
V
IN
IN
IN
(6c) For V
V
V
V
IN
IN
IN
(6b) For 2.5V < V
OUT
LT3688
LT3688
LT3688
(6a) For V
BIAS
BIAS
GND
BIAS
GND
GND
< 2.5V; V
BST
BST
BST
SW
SW
SW
OUT
IN(MAX)
OUT
> 2.8V
< 2.8V
C3
C3
C3
= 30V
D2
3688 F06
V
OUT
V
V
OUT
OUT
3688f