Datasheet

15
LT1307/LT1307B
1307fa
Externally Controlled Burst Mode Operation
This circuit overcomes the limitation of load-based
transitioning between Burst Mode operation and constant
switching mode by adding external control. If M1’s gate is
grounded by an external open-drain signal, the converter
functions normally in constant switching mode, delivering
3.3V. Output noise is low, however efficiency at loads less
than 1mA is poor due to the 1mA supply current of the
LT1307B. If M1’s gate is allowed to float, the low-battery
detector now drives the V
C
pin. R3 and R2 set the output
to 3V by allowing M1’s gate to go to V
OUT
until the output
voltage drops below 3V. R1 adds hysteresis, resulting in
low-frequency Burst Mode operation ripple voltage at the
output. By pulling the V
C
pin below a V
BE
, quiescent
current of the LT1307B drops to 60µA, resulting in accept-
able efficiency at loads in the 100µA range.
V
IN
V
C
SW
FB
LBO
LBI
LT1307B
L1
10µH
MBR0520
100k
2
CELLS
1nF
1µF
CERAMIC
1307 F19
GNDSHDN
C2*
10µF
CERAMIC
R3
698k
R4
1M
R2
49.9k
R5
590k
R1
10M
C1
100µF
V
OUT
3.3V
200mA
300k
V
OUT
M1
2N7002
GROUND = HIGH POWER/LOW NOISE
FLOAT = Burst Mode OPERATION
SHUTDOWN
3.0V IN LOW-POWER
Burst Mode OPERATION
C1 = AVX TPSC107K006R0150
L1 = COILCRAFT DO1608-103
SUMIDA CD43-100
C2 OPTIONAL: REDUCES OUTPUT
RIPPLE CAUSED BY C1'S ESR
*
+
0.2s/DIV 1307 F20
This photo details output voltage as the circuit is switched
between the two modes. Load current is 100µA in Burst
Mode operation; 10mA in constant switching mode.
This photo shows transient response in constant switch-
ing mode with a 10mA to 100mA stepped load. Output
ripple at the switching frequency can be reduced consid-
erably by adding a 10µF ceramic capacitor in parallel with
the 100µF tantalum.
10mA
100µA
I
L
V
OUT
500mV/DIV
2ms/DIV
1307 F21
V
OUT
100mV/DIV
100mA
10mA
I
L
TYPICAL APPLICATIO S
U