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LT1619

Low Voltage Current Mode PWM Controller

Linear Technology 

LT1619

U

OPERATIO

The LT1619 is a fixed frequency current mode switching

regulator PWM controller that can be used in boost, SEPIC

or flyback modes. The device operates from an input

supply range of 1.9V to 18V, and has a separate supply pin

(DRV) for the gate driver. The DRV pin can be bootstrapped

to VOUT for additional gate enhancement in low voltage

applications like 3.3V to 5V boost converters, or con-

nected to the input supply for higher voltage inputs.

To best understand operation of the LT1619, please refer

to Figure 2, the Block Diagram. The gate drive circuit turns

on the external MOSFET at the trailing edge of oscillator

output signal CLK. MOSFET current is sensed with an

external resistor (RSENSE of Figure 1). A leading edge

blanking circuit disables the current sense amplifier for

280ns immediately following switch turn-on, preventing

gate charging current from prematurely tripping the PWM

comparator. A slope compensating ramp, derived from

the oscillator, is added to the current sense output. The

driver turns off the MOSFET when this sum exceeds the

error amplifier output VC. The switch current is limited

with a separate comparator. The compensating ramp is a

progressive nonlinear function of the operating duty ratio

whereas the current limit does not vary with the duty ratio.

Error amplifier output VC determines the peak switch cur-

rent required to regulate the output voltage. VC can be

considered a measure of output current. At heavy loads,

VC is in its upper range. Average and peak inductor cur-

rents are high. In this range, the inductor tends to run in

continuous conduction mode (CCM), where current is al-

ways flowing in the inductor. As load current decreases,

average and peak inductor current decreases. When the

average inductor current falls below 1/2 of the peak-to-peak

inductor current ripple, the converter enters discontinu-

ous conduction mode (DCM), where current in the induc-

tor reaches zero sometime during the discharge phase.

Further reduction in output current moves VC towards its

lower operating range, decreasing inductor current. Hys-

teretic comparator A1 determines if VC is too low for the

LT1619 to operate efficiently. As VC falls below the trip

voltage VB, A1’s output goes high, turning off all blocks

except the error amplifier, A1 and A2. The LT1619 enters

the idle state and switching stops. The device draws just

140µA from the input in the idle state. Output load current

discharges the output capacitor, causing the output volt-

age to decrease. As VOUT decreases, VC increases. As VC

increases above VB, switching action begins, delivering

power to the output. The switch current sense threshold is

about 10mV in this VC region. If the output load remains

light, the output voltage will rise and VC will fall, causing

the converter to idle again. This is known as Burst Mode

operation. The burst frequency depends on input voltage,

output voltage, inductance and output capacitance. Out-

put voltage ripple during Burst Mode operation is usually

higher than when the converter is switching continuously.

Burst Mode operation increases light load efficiency be-

cause it delivers more energy per clock cycle than possible

with discontinuous mode operation and extremely low

peak switch current, allowing fewer switching cycles to

maintain a given output. IC supply current therefore be-

comes a small fraction of the total input current.

Setting Output Voltage

The output voltage of the LT1619 is set with resistive

divider R1 and R2 connected from the output to ground as

detailed in Figure 3. The divider tap is tied to the device FB

pin. Current through R2 should be significantly higher

than the FB pin bias current of 25nA. With R2 = 10k, the

input bias current of the error amplifier is 0.02% of the

current in R2.

LT1619

FB

VO

( ) R1

VO = 1.24V

1+

R1

R2

( ) R2

R1 = R2

VO

1.24

–1

1619 F03

Figure 3. Feedback Resistive Divider

Synchronization and Shutdown

The S/S pin (Pin 1) can be used to synchronize the

oscillator to an external source. The S/S pin is tied to the

input (VIN > 1.9V) for normal operation. The oscillator in

the LT1619 can be externally synchronized by driving the

S/S pin with a pulse train with an amplitude of at least 1V.

The maximum allowable rise time is a function of the

pulse amplitude, as shown in Table 1. Rise times equal to

6

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