LTC1558-3.3/LTC1558-5
APPLICATIONS INFORMATION
Once V
FB
rises above (V
REF
– 7.5%), the LTC1558’s boost
converter deactivates and the freshly charged input
capacitor supplies power to the system regulator. The
cycle repeats again when the input capacitor’s charge is
drained away and V
FB
again drops below (V
REF
– 7.5%).
The BACKUP pin remains asserted until the main battery
is restored. This ensures that the LTC1558 does not switch
in and out of backup mode unnecessarily.
The LTC1558’s boost converter minimizes output ripple
under light load conditions by reducing the charge trans-
ferred for the first two consecutive switch cycles. When
V
FB
falls below (V
REF
– 7.5%), the boost operation starts
by connecting the SW pin to ground through an internal
0.5Ω N-channel MOSFET (N1 in the Block Diagram). The
current through the external 22µH inductor rises linearly
through this switch.
330mA
(PEAK)
165mA
(PEAK)
LIGHT CURRENT MODE
HEAVY CURRENT MODE
1558 F02
Figure 2. Inductor Current During Switching
When the switch current reaches an internally preset level
of 165mA, the boost converter connects the SW pin to the
V
BAK
pin through an internal 2Ω P-channel MOSFET (P1 in
the Block Diagram). The inductor current discharges
through P1, charging up the capacitor connected exter-
nally to V
BAK
(C
IN
of the system regulator, Figure 1). The
inductor current falls at a rate proportional to the differ-
ence between the backup cell voltage and the output
voltage V
BAK
. When the inductor current reaches zero,
indicating all of its energy has been transferred to the
output capacitor, the LTC1558 looks at the FB pin voltage.
If V
FB
has increased above the (V
REF
– 7.5%) threshold,
the boost converter shuts off both switches and waits for
V
FB
to drop below (V
REF
– 7.5%) again.
If V
FB
is still less than (V
REF
– 7.5%) after the first boost
cycle, the LTC1558 immediately reconnects SW to ground,
repeating the boost cycle. If after two consecutive pulses,
V
FB
is still not above the boost threshold (V
REF
– 7.5%),
the LTC1558 decides that the load is not so light after all,
U
W
U
U
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 • LI
2
), doubling the available
output power. When V
FB
exceeds the (V
REF
– 7.5%) boost
threshold, the LTC1558 stops the boost converter and
resets the internal two pulse counter. The next time V
FB
falls below (V
REF
– 7.5%), the boost converter restarts in
low current mode for at least two boost cycles. Moderate
or changing loads will cause the LTC1558 to shift between
the two peak inductor current limits, keeping the output in
tight regulation. Near its maximum load capability, the
LTC1558 will stay in 330mA high current mode and the
output voltage V
BAK
will hover around the user pro-
grammed value.
V
BAK
Capacitor ESR
The type of output capacitor and the user programmed
V
BAK
value will affect the LTC1558’s output ripple and
efficiency. In most applications, the main V
BAK
capacitor
is primarily determined by the requirements of the main
power supply. Such a capacitor will generally meet the
requirements of the LTC1558. In unusual circumstances
or circuits where the main system regulator’s input ca-
pacitor is located some distance away from the LTC1558,
a local output capacitor may be necessary.
1
BOOST
CYCLE
V
BAK
ESR RIPPLE
DISCHARGE
PERIOD
t
DISCH
CHARGE
PERIOD
t
CH
1588 F03
Figure 3. V
BAK
Ripple
The maximum ripple on the V
BAK
pin is equal to 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
9
LINER [ LINEAR TECHNOLOGY ]
