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LT3758

3758f

APPLICATIONS INFORMATION

Main Control Loop

The LT3758 uses a ﬁxed frequency, current mode control

scheme to provide excellent line and load regulation. Op-

eration can be best understood by referring to the Block

Diagram in Figure 1.

The start of each oscillator cycle sets the SR latch (SR1) and

turns on the external power MOSFET switch M1 through

driver G2. The switch current ﬂows through the external

current sensing resistor RSENSE and generates a voltage

proportional to the switch current. This current sense

voltage VISENSE (ampliﬁed by A5) is added to a stabilizing

slope compensation ramp and the resulting sum (SLOPE)

is fed into the positive terminal of the PWM comparator A7.

When SLOPE exceeds the level at the negative input of A7

(VC pin), SR1 is reset, turning off the power switch. The

level at the negative input of A7 is set by the error ampliﬁer

A1 (or A2) and is an ampliﬁed version of the difference

between the feedback voltage (FBX pin) and the reference

voltage (1.6V or –0.8V, depending on the conﬁguration).

In this manner, the error ampliﬁer sets the correct peak

switch current level to keep the output in regulation.

The LT3758 has a switch current limit function. The current

sense voltage is input to the current limit comparator A6.

If the SENSE pin voltage is higher than the sense current

limit threshold VSENSE(MAX) (110mV, typical), A6 will reset

SR1 and turn off M1 immediately.

The LT3758 is capable of generating either positive or

negative output voltage with a single FBX pin. It can

be conﬁgured as a boost, ﬂyback or SEPIC converter

to generate positive output voltage, or as an inverting

converter to generate negative output voltage. When

conﬁgured as a SEPIC converter, as shown in Figure 1,

the FBX pin is pulled up to the internal bias voltage of 1.6V

by a voltage divider (R1 and R2) connected from VOUT to

GND. Comparator A2 becomes inactive and comparator

A1 performs the inverting ampliﬁcation from FBX to VC.

When the LT3758 is in an inverting conﬁguration, the

FBX pin is pulled down to –0.8V by a voltage divider

connected from VOUT to GND. Comparator A1 becomes

inactive and comparator A2 performs the noninverting

ampliﬁcation from FBX to VC.

The LT3758 has overvoltage protection functions to

protect the converter from excessive output voltage

overshoot during start-up or recovery from a short-circuit

condition. An overvoltage comparator A11 (with 20mV

hysteresis) senses when the FBX pin voltage exceeds the

positive regulated voltage (1.6V) by 8% and provides a

reset pulse. Similarly, an overvoltage comparator A12

(with 10mV hysteresis) senses when the FBX pin voltage

exceeds the negative regulated voltage (–0.8V) by 11%

and provides a reset pulse. Both reset pulses are sent to

the main RS latch (SR1) through G6 and G5. The power

MOSFET switch M1 is actively held off for the duration of

an output overvoltage condition.

Programming Turn-On and Turn-Off Thresholds with

the SHDN/UVLO Pin

The SHDN/UVLO pin controls whether the LT3758 is

enabled or is in shutdown state. A micropower 1.22V

reference, a comparator A10 and a controllable current

source IS1 allow the user to accurately program the sup-

ply voltage at which the IC turns on and off. The falling

value can be accurately set by the resistor dividers R3

and R4. When SHDN/UVLO is above 0.7V, and below the

1.22V threshold, the small pull-down current source IS1

(typical 2μA) is active.

The purpose of this current is to allow the user to program

the rising hysteresis. The Block Diagram of the comparator

and the external resistors is shown in Figure 1. The typical

falling threshold voltage and rising threshold voltage can

be calculated by the following equations:

VµA

VIN FALLING

VIN RISING

.•

()

•

122

IN FALLING

For applications where the SHDN/UVLO pin is only used

as a logic input, the SHDN/UVLO pin can be connected

directly to the input voltage VIN through a 1k resistor for

always-on operation.

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