LTC1559CS-5 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1559CS-5' PDF : 20 Pages View PDF
LTC1559-3.3/LTC1559-5
APPLICATIONS INFORMATION
If VCC is still less than VCC (rated value) – 7% (LTC1559-
3.3) after the first boost cycle, the LTC1559 immediately
reconnects SW to ground, repeating the boost cycle. If
after two consecutive pulses, VCC is still not above the
boost threshold VCC (rated value) – 7% (LTC1559-3.3),
the LTC1559 decides that the load is not so light after all,
and doubles the internal inductor charging current limit to
330mA for subsequent cycles. This is high current mode.
By doubling the peak inductor current, each boost cycle
effectively carries four times more energy compared to
low current mode (E = 1/2 • LI2), doubling the available
output power. When VCC exceeds the VCC (rated value)
– 7% (LTC1559-3.3) boost threshold, the LTC1559 stops
the boost converter and resets the internal 2-pulse counter.
The next time VCC falls below VCC (rated value) – 7%
(LTC1559-3.3), the boost converter restarts in low current
mode for at least two boost cycles. Moderate or changing
loads cause the LTC1559 to shift between the two peak
inductor current limits, keeping the output in tight regula-
tion. Near its maximum load capability, the LTC1559 will
stay in 330mA high current mode and the output voltage
VBAK will hover around VCC (rated value) – 7%
(LTC1559-3.3).
VCC Capacitor ESR
The type of output capacitor and the rated VCC value will
affect the LTC1559’s output ripple and efficiency. In most
applications, the VCC capacitor is primarily determined by
the requirements of the main power supply. Such a
capacitor will generally meet the requirements of the
LTC1559. In unusual circumstances or circuits where
the main system VCC capacitor is located some distance
away from the LTC1559, a local output capacitor may be
necessary.
The ripple on the VCC pin is equal to the capacitor ESR
voltage drop due to the boost converter’s output current
pulses. The ripple frequency and output duty cycle is
proportional to the inductor discharge time. Given a fixed
inductor value (22µH) and a known peak current limit, the
booster’s discharge time in each boost cycle is propor-
tional to the difference between VBAK (93% of the rated VCC
for the LTC1559-3.3 and 92.5% of the rated VCC for the
LTC1559-5) and the battery cell voltage, VBAT (1.2V).
Assuming ESR = 0.2Ω, IIND(PEAK) = 330mA, VCC = 5V,
VRIPPLE(P-P) = (IIND(PEAK))(RESR(CAP))
= (330mA)(0.2Ω)
= 66mV
Since VCC must be scaled down internally, the external
resistor ratio:
= 5V/1.272V
= 3.931
Therefore the ripple seen by the VCC comparators is:
= 66mV/3.931
= 16.79mV
The discharge time period,
tDISCH = (L • IIND(PEAK))/(VBAK – VBAT)
= (22µH • 330mA)/(4.625 – 1.2V)
= 2.12µs
For VCC = 3.3V and IIND(PEAK) = 330mA,
VRIPPLE(P-P) = 66mV
RB resistor ratio = 3.3/1.272 = 2.594
Ripple voltage = 25.4mV
tDISCH = 3.9µs
The internal VCC comparators are designed to have a slow
response time to filter away this ripple. The VCC (rated
value) – 5.5% (LTC1559-3.3) and VCC(rated value) – 9%
comparators have a 6µs rising edge delay and 2µs falling
edge delay. The VCC (rated value) – 7% (LTC1559-3.3)
comparator has a similar 6µs rising time delay but a much
longer falling time delay of 20µs. This enables the com-
parator to control the boost converter properly, and avoids
turning off the boost converter prematurely due to false
triggering by the ESR ripple.
Exit from Backup
When a main battery is inserted into the system, the
LTC1559 follows a specific sequence to exit backup mode
and return control to the main supply. The sequence
depends on the type of main power supply used. In
systems where the main supply’s output impedance is
10
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