LTC3736-1 Datasheet, PDF (11/28 Pages) Linear Technology – Dual 2-Phase, No RSENSE Synchronous Controller with Spread Spectrum

U
OPERATIO (Refer to Functional Diagram)
operation enabled) with V_IN = 5V, V_OUT = 2.5V and I_OUT =
1A. Note the significant reduction in peak output noise
(>20dBm).
Short-Circuit Protection
When an output is shorted to ground (V_FB < 0.12V), the
switching frequency of that controller is reduced to 1/5 of
the normal operating frequency. The other controller is
unaffected and maintains normal operation.
The short-circuit threshold on V_FB2 is based on the smaller
of 0.12V and a fraction of the voltage on the TRACK pin.
This also allows V_OUT2 to start up and track V_OUT1 more
easily. Note that if V_OUT1 is truly short-circuited
(V_OUT1 = V_FB1 = 0V), then the LTC3736-1 will try to
regulate V_OUT2 to 0V if a resistor divider on V_OUT1 is
connected to the TRACK pin.
LTC3736-1
Output Overvoltage Protection
As a further protection, the overvoltage comparator (OV)
guards against transient overshoots, as well as other more
serious conditions that may overvoltage the output. When
the feedback voltage on the V_FB pin has risen 13.33%
above the reference voltage of 0.6V, the external P-chan-
nel MOSFET is turned off and the N-channel MOSFET is
turned on until the overvoltage is cleared.
Frequency Selection (FREQ Pin)
(Spread Spectrum Operation Disabled)
The switching frequency of the LTC3736-1 can be selected
using the FREQ pin when spread spectrum operation is
disabled (SSDIS = V_IN).
–10
R_BW = 3kHz
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
0
6M 12M 18M 24M 30M
FREQUENCY (Hz)
37361 F01a
Figure 1a. Output Noise Spectrum of Conventional Buck
Switching Converter (LTC3736-1 with Spread Spectrum
Disabled) Showing Fundamental and Harmonic Frequencies
–10
R_BW = 3kHz
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
0
6M 12M 18M 24M 30M
FREQUENCY (Hz)
37361 F01c
Figure 1c. Output Noise Spectrum of the LTC3736-1 Spread
Spectrum Buck Switching Converter. Note the Reduction in
Fundamental and Harmonic Peak Spectral Amplitude
Compared to Figure 1a.
–10
R_BW = 30Hz
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
410k 450k 490k 530k 570k 610k
FREQUENCY (Hz)
37361 F01b
Figure 1b. Zoom-In of Fundamental Frequency of Conventional
Buck Switching Converter
–10
R_BW = 30Hz
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
410k 450k 490k 530k 570k 610k
FREQUENCY (Hz)
37361 F01d
Figure 1d. Zoom-In of Fundamental Frequency of the
LTC3736-1 Spread Spectrum Switching Converter. Note the
>20dB Reduction in Peak Amplitude and Spreading of the
Frequency Spectrum (Between Approximately 450kHz and
580kHz) Compared to Figure 1b.
37361f
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