ltm8008 Linear Technology Corporation, ltm8008 Datasheet - Page 8

• Manufacturer Part Number: ltm8008
• Description: Ltm8008 - 72vin, 6 Output Dc/dc Sepic ?module Regulator
• Manufacturer: Linear Technology Corporation

LTM8008
applicaTions inForMaTion
Running Input Voltage Versus Start Voltage
In normal operation the LTM8008 SEPIC Converter output
SPV is used to run the INTV
a rectifying diode (see the Block Diagram). This allows
the internal MOSFET driver Q1 and other circuitry to be
properly biased even when the input voltage falls as low
as 3V. At start-up, however, the SPV voltage is low and
INTV
order to properly bias the LTM8008 internal circuitry at
start-up, V
Main Control Loop
The LTM8008 uses a fixed frequency, current mode con-
trol scheme to provide excellent line and load regulation.
At the start of each oscillator cycle, a latch turns on the
internal power MOSFET switch. The switch current flows
through an internal current sensing resistor and generates
a voltage proportional to the switch current. This current
sense voltage is added to a stabilizing slope compensation
ramp and the resulting sum is compared with the voltage
on the V
exceeds the level of the V
turning off the power switch. The level at the V
amplified version of the difference between the feedback
voltage and the reference voltage. In this manner, the error
amplifier sets the correct peak switch current level to keep
the output in regulation. The LTM8008 is also equipped
with a switch current limit function. If the detected current
is higher than 8.2A, typical, the internal circuitry will turn
off Q1 for the rest of the switching cycle.
The LTM8008 has overvoltage protection functions to pro-
tect the converter from excessive output voltage overshoot
during start-up or recovery from a short-circuit condition.
If the output voltage exceeds the targeted set point by
8%, internal circuitry will actively inhibit switching for the
duration of an output overvoltage condition.
Programming Turn-On and Turn-Off Thresholds with
the RUN Pin
The RUN pin controls whether the LTM8008 is enabled
or shut down. Low power circuitry allows the user to ac-
curately program the supply voltage at which the SEPIC
converter turns on and off. The falling value can be ac-
8
CC
is derived from V
C
pin. When the stabilized current sense voltage
IN
must rise to at least 6V.
IN
C
pin, the internal latch is reset,
through a linear regulator. In
CC
internal biasing through
C
pin is an
curately set by the resistor divider network composed of
R3 and R4 shown in Figure 1.
If RUN is above 1.21V, the LTM8008 is running. Below
1.21V and above 0.7V, the LTM8008 is off and the RUN
pin sinks a hysteresis current (typically 2μA). Below 0.7V,
the LTM8008 is not switching and is in a low power state,
drawing less than 70µA at V
voltage and rising threshold voltage can be calculated by
the following equations:
For applications where the RUN pin is not used, the RUN
pin can be connected directly to the input voltage INTV
for always-on operation.
INTV
An internal, low dropout (LDO) voltage regulator produces
the INTV
start-up or whenever SPV is low. The LTM8008 contains
an undervoltage lockout comparator and an overvoltage
lockout comparator for the INTV
undervoltage (UV) function protects the internal circuitry
from attempting to operate in a brown-out condition, while
the overvoltage (OV) function protects the gate of the
power MOSFET and excessive power dissipation within
the LTM8008 in the case of a fault. When INTV
the UV threshold, or above the OV threshold, switching
stops and the soft-start operation will be triggered.
The INTV
mediately adjacent to the LTM8008 pins with a minimum
of a 4.7μF ceramic capacitor. Good bypassing is neces-
sary to supply the high transient currents required by the
MOSFET gate driver.
Figure 1. The LTM8008 Turn-On and Turn-Off Thresholds Are Set
by a Simple Resistor Network
V
V
VIN,FALLING
VIN,RISING
CC
Regulator Bypassing and Operation
CC
CC
supply which powers the internal circuitry during
regulator must be bypassed to ground im-
= 2µA • R3 + V
= 1.21•
R3
R4
V
IN
RUN
(
R3 + R4
IN
LTM8008
R4
. The typical falling threshold
IN,FALLING
)
CC
8008 F01
supply. The INTV
CC
is below
8008fa
CC
CC