LT3579 LINER [Linear Technology], LT3579 Datasheet - Page 11

• Manufacturer Part Number: LT3579
• Description: 6A Boost/Inverting DC/DC Converter with Fault Protection
• Manufacturer: LINER [Linear Technology]

OPERATION
Sample Mode
Sample Mode is the mechanism used by the LT3579 to
aid in the detection of output shorts. It refers to a state of
the LT3579 where the master and slave power switches
(Q1 and Q2) are turned on for a minimum period of time
every clock cycle (or every few clock cycles in frequency
foldback) in order to “sample” the inductor current. If the
sampled current through Q1 exceeds the master switch
current limit of 3.4A (minimum), the LT3579 triggers an
overcurrent fault internally (see Operation-Fault section for
details). Sample Mode exists when FB is out of regulation
by more than 3.7% or 45mV < FB < 1.17V (typical). The
LT3579’s power switches are designed to handle a total
peak current of 6A (minimum).
Frequency Foldback
The frequency foldback circuit reduces the switching
frequency when 350mV < FB < 900mV (typical). This
feature lowers the minimum duty cycle that the part can
achieve, thus allowing better control of the inductor current
during start-up. When the FB voltage is pulled outside of
this range, the switching frequency returns to normal.
Note that the peak inductor current at start-up is a
function of many variables including load profi le, output
capacitance, target V
the application’s performance at start-up to ensure that
the peak inductor current does not exceed the minimum
current limit.
OPERATION – REGULATION
The following description of the LT3579’s operation
assumes the FB voltage is close enough to its regulation
target so that the part is not in Sample Mode. Use the
Block Diagram as a reference when stepping through
the following description of the LT3579 operating in
regulation. At the start of each oscillator cycle, the SR
latch (SR1) is set, which turns on the power switches Q1
and Q2. The collector current through the master switch,
Q1, is ~1.3 times the collector current through the slave
switch, Q2, when the collectors of the two switches are
tied together. Q1’s emitter current fl ows through a current
OUT
, V
IN
, switching frequency, etc. Test
sense resistor (R
the total switch current. This voltage (amplifi ed by A4) is
added to a stabilizing ramp and the resulting sum is fed
into the positive terminal of the PWM comparator A3.
When the voltage on the positive input of A3 exceeds
the voltage on the negative input, the SR latch is reset,
turning off the master and slave power switches. The
voltage on the negative input of A3 (V
(or A2), which is simply an amplifi ed difference between
the FB pin voltage and the reference voltage (1.215V if
the LT3579 is confi gured as a boost converter, or 9mV
if confi gured as an inverting converter). In this manner,
the error amplifer sets the correct peak current level to
maintain output regulation.
As long as the part is not in fault (see Operation – FAULT
section) and the SS pin exceeds 1.8V, the LT3579 drives its
CLKOUT pin at the frequency set by the RT pin or the SYNC
pin. The CLKOUT pin can synchronize other compatible
switching regulator ICs (including additional LT3579s) with
the LT3579. Additionally, the duty cycle of CLKOUT varies
linearly with the part’s junction temperature and may be
used as a temperature monitor. The CLKOUT signal on the
LT3579-1 is ~180° out of phase with the internal oscillator
and has a fi xed duty cycle of ~50%.
OPERATION – FAULT
The LT3579’s FAULT pin is an active low, bidirectional
pin (refer to Block Diagram) that pulls low to indicate a
fault. Each of the following events can trigger a fault in
the LT3579:
A. FAULT1 Events:
B. FAULT2 Events:
1. SW Overcurrent
2. V
3. SW1 Voltage and/or SW2 Voltage > 42V
4. Die Temperature > 165°C
1. Pulling the FAULT pin low externally
a. I
b. (I
(minimum)
IN
SW1
Voltage > 16.2V (minimum)
SW1
> 3.4A (minimum)
+ I
S
SW2
) generating a voltage proportional to
) > 6A (minimum)
LT3579/LT3579-1
C
pin) is set by A1
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