Hig h -Effic ie n c y, P WM, S t e p -Do w n
DC-DC Co n t ro lle rs in 1 6 -P in QS OP
2–MAX165
output voltage as determined by the turns ratio is +15V,
input surge currents, and perhaps for power-supply
sequencing. In shutdown mode, the soft-start circuit
holds the SS capacitor discharged to ground. When
SHDN goes high, a 4µA current source charges the SS
capacitor up to 3.2V. The resulting linear ramp wave-
form causes the internal current-limit level to increase
proportionally from 0 to 100mV. The main output capaci-
tor thus charges up relatively slowly, depending on the
SS capacitor value. The exact time of the output rise
depends on output capacitance and load current and is
typically 1ms per nanofarad of soft-start capacitance.
With no SS capacitor connected, maximum current limit
is reached within 10µs.
the feedback resistor ratio should be set to produce
about +13.5V; otherwise, the SECFB one-shot might be
trig g e re d uninte ntiona lly, c a us ing a n unne c e s s a ry
increase in supply current and output noise. In negative-
output (MAX1654) applications, the resistor-divider acts
as a load on the internal reference, which in turn can
cause errors at the main output. Avoid overloading REF
(see the Reference Load-Regulation Error vs. Load
Current graph in the Typical Operating Characteristics).
100kΩ is a good value for R3 in MAX1654 circuits.
Output current on secondary winding applications is
limite d a t low inp ut volta g e s . Se e the MAX1652
Maximum Secondary Output Current vs. Supply Voltage
graph in the Typical Operating Characteristics for data
from the application circuit of Figure 8.
S h u t d o w n
Shutdown mode (SHDN = 0V) reduces the V+ supply
current to typically 3µA. In this mode, the reference and
VL are inactive. SHDN is a logic-level input, but it can
be safely driven to the full V+ range. Connect SHDN to
V+ for automatic start-up. Do not allow slow transitions
(slower than 0.02V/µs) on SHDN.
S o ft -S t a rt Circ u it (S S )
Soft-start allows a gradual increase of the internal cur-
rent-limit level at start-up for the purpose of reducing
22µF, 35V
V (6.5V TO 18V)
IN
22µF, 35V
+15V AT
250mA
C2
4.7µF
D1
210k, 1%
49.9k, 1%
2
7
11
VL
CMPSH
-3A
C2
4.7µF
C3
15µF
2.5V
SECFB
FB
10
V+
Si9410
16
14
DH
0.01µF
18V
1/4 W
D2
EC11FS1
6
ON/OFF
SHDN
BST
+5V
0.1µF
AT 3A
15
T1
LX
MAX1652
20mΩ
15µH
2.2:1
220µF
10V
220µF
10V
Si9410
13
12
1
4
1N5819
DL
SS
PGND
0.01µF
(OPTIONAL)
8
9
GND
CSH
CSL
22Ω*
4700pF*
SYNC
REF
T1 = TRANSPOWER TTI5870
* = OPTIONAL, MAY NOT BE NEEDED
5
3
0.33µF
Figure 8. 5V/15V Dual-Output Application Circuit (MAX1652)
______________________________________________________________________________________ 19
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