LT1307CS8 datasheet(8/20 Pages) LINER

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LT1307/LT1307B

APPLICATIONS INFORMATION

OPERATION

The LT1307 combines a current mode, fixed frequency

PWM architecture with Burst Mode micropower operation

to maintain high efficiency at light loads. Operation can

best be understood by referring to the block diagram in

Figure 2. Q1 and Q2 form a bandgap reference core whose

loop is closed around the output of the converter. When

VIN is 1V, the feedback voltage of 1.22V, along with an

80mV drop across R5 and R6, forward biases Q1 and Q2’s

base collector junctions to 300mV. Because this is not

enough to saturate either transistor, FB can be at a higher

voltage than VIN. When there is no load, FB rises slightly

above 1.22V, causing VC (the error amplifier’s output) to

decrease. When VC reaches the bias voltage on hysteretic

comparator A1, A1’s output goes low, turning off all

circuitry except the input stage, error amplifier and low-

battery detector. Total current consumption in this state is

µA. As output loading causes the FB voltage to de-

crease, A1’s output goes high, enabling the rest of the IC.

Switch current is limited to approximately 100mA initially

after A1’s output goes high. If the load is light, the output

voltage (and FB voltage) will increase until A1’s output

goes low, turning off the rest of the LT1307. Low fre-

quency ripple voltage appears at the output. The ripple

frequency is dependent on load current and output capaci-

tance. This Burst Mode operation keeps the output regu-

lated and reduces average current into the IC, resulting in

high efficiency even at load currents of 100

µA or less.

If the output load increases sufficiently, A1’s output re-

mains high, resulting in continuous operation. When the

LT1307 is running continuously, peak switch current is

controlled by VC to regulate the output voltage. The switch

is turned on at the beginning of each switch cycle. When

the summation of a signal representing switch current and

a ramp generator (introduced to avoid subharmonic oscil-

lations at duty factors greater than 50%) exceeds the VC

signal, comparator A2 changes state, resetting the flip-

flop and turning off the switch. Output voltage increases as

switch current is increased. The output, attenuated by a

resistor divider, appears at the FB pin, closing the overall

loop. Frequency compensation is provided by an external

series RC network connected between the VC pin and

ground. Low-battery detector A4’s open collector output

(LBO) pulls low when the LBI pin voltage drops below

200mV. There is no hysteresis in A4, allowing it to be used

as an amplifier in some applications. The entire device is

disabled when the SHDN pin is brought low. To enable the

converter, SHDN must be at VIN or at a higher voltage.

The LT1307B differs from the LT1307 in that there is no

hysteresis in comparator A1. Also, the bias point on A1 is

set lower than on the LT1307 so that switching can occur

at inductor current less than 100mA. Because A1 has no

hysteresis, there is no Burst Mode operation at light loads

and the device continues switching at constant frequency.

This results in the absence of low frequency output voltage

ripple at the expense of efficiency.

The difference between the two devices is clearly illus-

trated in Figures 3 and 4. The top two traces in Figure 3

show an LT1307/LT1307B circuit, using the components

indicated in Figure 1, set to a 5V output. Input voltage is

1.25V. Load current is stepped from 1mA to 41mA for both

circuits. Low frequency Burst Mode operation voltage

ripple is observed on Trace A, while none is observed on

TRACE A

TRACE B

LT1307

VOUT

500mV/DIV

AC COUPLED

41mA

1mA

LT1307B

VOUT

500mV/DIV

AC COUPLED

VIN = 1.25V

1ms/DIV

1307 F03

VOUT = 5V

Figure 3. LT1307 Exhibits Burst Mode Operation Ripple at

1mA Load, LT1307B Does Not

LT1307

VOUT

200mV/DIV

AC COUPLED

TRACE A

45mA

5mA

LT1307B

VOUT

200mV/DIV

AC COUPLED

TRACE B

VIN = 1.5V

500

µs/DIV

1307 F04

VOUT = 5V

Figure 4. At Higher Loading and a 1.5V Supply, LT1307

Again Exhibits Burst Mode Operation Ripple at 5mA Load,

LT1307B Does Not

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